In regards to the games, there's no such thing as hard or easy. Grind enough levels, and the games WILL be easy. I think the idea refers to level curve, and that itself is arbirtary. If you look at RBY, it's incredibly easy to beat the game with a team of Lv. 40 Pokémon... you just need to use TMs to get better moves. In BW, there are enough trainers to have you ebat the game with a team of Pokémon that are equal level to the final trainers. As a hacker, if you're always throwing out Pokémon that are higher level than the player's, the player's Pokémon will just level up faster. So as a hack, I think it'd be important to have boss opponents that have well-crafted teams, forcing you to think of how you'll set up your team, as opposed to being able to plow through a game with a team of underlevelled Pokémon that outdo the opponents because of more powerful moves.
So to answer the question, it depends on how you want the hack to be. If you look at GSC, the games are much more open-ended compared to the other games, with a lot of optional dungeons thrown at you after the 4th Gym. (Mt. Mortar, Burned Tower in GS, Whirl Islands a bit after, Dark Cave, additional sections of Union Cave and Slowpoke Well, etc...). All of those optional areas follow the exact same levels for wild Pokémon/Trainers but all yield different species as to keep things fresh. Of course, the later games had tons of optional areas too (save for the linear fest that is BW, with only 1.5 optional areas before the E4).
As for me, I like having to explore every area in the game in sequence, but in a way more akin to Sinnoh, where you revisit older areas to access new ones. A mudslide north of Oreburgh prevents you from accessing Mt. Coronet, but you can pass that obstacle after getting that bike, and then that idea is introduced again when you loop back to Hearthome to fight Fantina (in DP, anyways). The last 3 gyms are all on the edges of Sinnoh, just past familiar areas. tl;dr you've explored the majority of Sinnoh by the 5th Gym, but still with a few optional areas along the way with awesome rewards (such as Iron Island's reward of the Riolu Egg and a Dawn Stone I believe).
So, uh, sorry for that kinda in-depth analysis of region design and difficulty, but I feel it's necessary to factor that in when considering difficulty. I personally believe that the main series games all have about the same difficulty, with DP possibly being the most difficult due to the level spike before the E4. BW's introduction of reusable TMs allowed Game Freak allowed them to have more elaborate trainers with more varied Pokémon. In my playthrough, I had a team with a very balanced moveset without worrying about wasting a TM on a female, AND a perfectly levelled team because of the trainers' level curve. In RBY, I have an underlevelled team with moves covering their weaknesses, which allowed me to plow through all the mandatory trainers while being 15 levels below.
Lots of stuff to consider.