I'm going to really enjoy giving my two cents here, because I think gen 5 is pretty awesome too. I don't think people do hate these games actually.
What I have found by communicating with fans is that BW2 is often regarded as one of the best, if not the best game in the main series. Even those who may have reservations about the original Black White games felt the sequels corrected previous mistakes and offered an enjoyable experience. Since its release many people have wanted to see sequel games again and were disappointed that gen 7 did not follow in BW2's footsteps. BW2's post-game content is also often hailed as raising the bar to new levels like HGSS.
Now the original Black and White is interesting to discuss. People love or hate it. I think its the Vincent Van Gogh of the pokemon games, you could not sell it to everybody at the time, but the admirers it had were always devoted, and now with time public opinion is on its side, and it is remembered and loved with posterity.
I myself fell in love with Pokemon Black/ White at first sight, literally from the opening title scene with N's coronation I was hooked. It overnight became my favorite of the main series games at that time, and I even hold the less popular opinion that the original is even better than the sequel. I just loved what Black/White was doing with character through the anti-villain N, attempting a more epic story in scope with a score that was operatic to my ears and beautiful changes of seasons in the overworld
Yet, despite my own enthusiasm I can see why it was a controversial game. For one Unova was the first region not inspired by Japan. It changed the feel of the environment dramatically after the tradition maintained in the 4 previous games. The subsequent generations 6 and 7 made this choice as well, and have also fought for acceptance by fans. While I don't think it was solely due to this that there were fans who could not relate to gen 5, I would not underestimate this as a possible influence. Unova with its inspiration drawn from the United States, more specifically New York was a different face of the Pokemon series we know and love.The cities looked more modern, and less historic, and it looked very imposing, and may not have granted the same intimacy.
Where they fall on your list of favorites may be debated, but I have not really come across gens 1-4 spoken of with nearly the controversy of 5-7, and I think setting may be a factor, because we aren't always ready for change. Something can be great, but when it is just not what you are used to then you may not like it, even if you can't always put your finger on why.
Unova can also borderline on feeling surreal, its vast and dark. There are long stretches of space as we cross bridges, the graphics pushed the ds to the limits with cut scenes and towering buildings. Castelia could even become a confusing place to navigate with twists and turns down alleyways, npcs running toward toward the screen. Also the gyms of Unova are rarely in organic environments, nothing like Erika's simple garden. These gyms have roller coasters in them, restaurants and canons. The imagination of the gyms was one of my favorite parts, but cumulatively I think all of these things lead to Unova feeling really "out there " and potentially left some fans unable to relate with this world when compared with previous games.
BW also removed some features from previous games. Since generation 4 introduced the very popular mechanic of Pokemon following you, some fans, myself included, would have liked to have seen that implemented in future games, while no other generation has this, with gen 5 happened to be to break it to us that the party was over, since it came directly after gen 4. It als phased our Pokemon contests, and the battle frontier, and removed the game corner for good-- not even diluted Voltorb flip. Though the sequels compensated by adding many dazzling new features like the battle tree, the world tournament, pokestar studios, join avenue, the nature preserve, nobody had a crystal ball and knew at the time of Black and White that all of these innovations were coming. Outside of the main story game play was relatively limited with the choice being pretty much musicals or the battle subway. I think this contributed to people looking at Unova more critically initially, but favorably now that the 5th generation is complete and you can see the whole picture.
Though there has been some criticism of the Pokemon is gen 5 as having the bad designs, I think that is very unfair, and one of the weakest arguements that has been used against gen 5. It is fortunate when you have *this many* good Pokemon. It had Volcarona, Hydreigon, Victini, Haxorus, Excadrill, Braviary, all of the starters in the first stage of evolution are also cute, Krookodolie was a well-received pokemon too.
Every generation always has Pokemon that are not going to be fan favorites. Judge gen 1 by Mr. Mime, Electrode and Jynx? Judge Gen 4 by Probopass or Purugly?
Garbador gets held up as the example of why gen 5 sucks, but for every Garbador there was a striking-looking and powerful pokemon like Zekrom, for every generic elemental monkey there is a delightful creature like Sawsbuck, Whimsicott or Lilligant, and for every failed idea like Vanilluxe there was a successful one like Chandelure. Gen 5 not only made some good Pokemon, it made a *lot*of Pokemon, the amount of work that went into these games is really quite impressive, and it had dynamic new battle and sprite animations for the pokemon and even the trainer in addition to overhauling the Dex with quantity and diversity.
I really don't think it is that the Pokemon of Unova are bad at all, I think what frustrated fans about the pokemon was their unfamiliarity because there were no Pokemon from previous generations available until post game in Black/ White. It made the games feel alien to some, and was more of a challenge to play.
I don't think it would have mattered what Pokemon were in the Dex. It was just that they were all new Pokemon. I personally was fine with the restriction to use only new Pokemon in the first game and the opportunity to slowly explore and find out the best pokemon through trial and error, rather than rely on the crutch of my old favorites. I love a challenge.
I think ultimately the wild card with fans is the story of the original. Again, the ambitious story is precisely why I love it, but I have listened to a variety of opinions and some fans lost interest in the plot, calling it "talky" and moralizing.
I have said before in comments, that I see gen 6 in many ways as anti-Unova. It did almost everything opposite of 5. Unova is very serious in tone with darker characters and backstory, unusual, avant-garde Pokemon and a story so large and complex in scope that it spanned years and continued into a sequel. However, gen 6 is a short, simpler story with more light-hearted characters. It seeks to take us out of the dark and into light, and we go back to basics as far as the pokemon are concerned as well, introducing fewer pokemon to keep track of this time around while keeping a firm connection to previous regions by making Pokemon from older generations readily available from the get-go and updating old favorite Pokemon with mega evolutions. Whereas gen 5 was criticized for feeling isolated as a region and having 'ugly pokemon," gen 6 I feel really made a conscious effort to focus on making the game as beautiful and busy as possible, with warm, whimsical landscapes, the largest team of rivals yet so that you are almost never alone, and intimate close-ups and experiences with your Pokemon and fun mini games through poke amie. It felt very reactionary to gen 6 criticism.
However, though I like gen 6 too, it is also a game that divided fans, while I have heard some voices say initially gen 6 was the step in the right direction and see it as more fun experience with much better Pokemon than 5. More often I hear XY called superficial and easy, and even directly compared unfavorably with gen 5, citing gen 5 as having a better story and characters, more Pokemon to choose from, offering more challe ge and an elaborate post game. Many people missed gen 5. They continued to make negative comparisons with gen 7 as failing to live up to gen 5 despite its efforts to concentrate more on story like the Unova games.
Personally I think all core series games gen 1-7 have something good about them, and I don't want to make whether or not the criticism of the later games are correct the focus of this comment. The point I want to make is that many fans have come to love and appreciate gen. I think BW was ahead of its time. Though Black and White may have seemed like strange games at time, now that the dust has settled, and the newness of it is no longer here, many now praise the focus on story and its characters, the message, the strong villain with Ghetsis and tough final boss fight, the elaborate post-game content,ground breaking new items, moves, reuseable tms, the introduction of dreamworld abilities that gave some Pokemon a fresh start, and and overall originality and expect other games to be like it. It has risen from experiemcing alot of blaklash and become one of the most popular gens
To my surprise and delight when pokecommunity polled members on their favorite generation I believe it was in the lead, or came very close.
Though it had a rocky start, i think gen 5 will be rembered as great and revolutionary, and is already on its way.