It's entirely fair. One thing I will never understand is why people downplay USUM's additions in comparison to every other third game. I'm saying this as someone who's first generation was third gen and my favourite is fourth, that Emerald and Platinum introduced absolutely nothing "significant" to RS. I guess if you count the slight storyline change in including Rayquaza and Giratina, but USUM did the very same thing with Necrozma. Battle Frontier got its own island in Emerald and Platinum, but that means very little in the grand scheme of things. DPPt introduced to us The Underground which is the best thing ever, but Mantine Surfing is a very quick and efficient (arguably fun depending on what side of the argument you're on) and not to mention easy way to earn BP for move tutors, when prior to this you just hand to mindlessly battle your way in previous games in order to get any decent amount of BP whatsoever (even then you earned very little, making getting any worthwhile moves a chore). USUM, Emerald, and Platinum are basically all in the same boat because they've all done essentially the same thing -- slight storyline changes, USUM added a mini-game while Platinum didn't really do much of anything in the way of that and neither did Emerald. All the latter two had going for it was the Battle Frontier and Rayquaza.
Honestly I'd just argue Yellow had the most changes out of every third game, but that's mostly because it changes the very fundamentals of the game to a degree due to who your starter is. Crystal I'll grant you has a significant change in adding Kris as the first female playable character and introducing Battle Tower for the first time, but the gameplay is arguably the same as GS and that's my bigger point that you're missing here: ever third game (
maybe bar Yellow version) feels largely the same to the prior two games that were released. The whole point of third games is essentially to serve as some sort of retelling of the first two games in addition to some sort of minor feature addition, some storyline changes, and maybe some other changes here and there. That was always the trend since Crystal and it holds true to this day no matter how you look at it with nostalgia goggles.
You can debate against anyone all day and night about the
quality of the additions USUM included, but I'm not arguing quality, it's
quantity. As far as quantity is concerned, it's just about the same (or around there) as every other third game released.
By the way, I agree that marketing the games as something entirely different was a huge flaw, but I don't expect TPCi to undersell a Pokemon game. A marketing company's job is to hype up the product they're trying to sell, after all.