Pretty much every single criticism of this game that I've seen is something that I would've never noticed while playing, in comparison to other Pokemon games where there's always that one major flaw that plagues the entire run. Hell, all the main criticisms of this game are points that I think it does a lot better than other games in the series.
>Level curve
Although not perfect, it had the right idea. It is true that there's a few bosses that get steamrolled (most notable Pryce), but at the game's highest points, when my team is at lower levels than the opponent's, I see myself using actual strategy instead of spamming my strongest attack.
>Pokemon distribution
This one I just don't get at all. I have a completely different team almost every time I play through HGSS. You get a ton of options even before the first gym, and you're still finding new Pokemon at the end of the game. There's even a good few mons made available much earlier than they were in GSC. I'd argue HGSS has some of the best Pokemon distribution in the series.
>Kanto is gutted
No, not really. In GSC, absolutely, although I understand why it kind of had to be. But in HGSS? In fact, the actual maps are massive improvements over FRLG, and there's tons of Pokemon to catch, tons of trainers to fight, it's great.
>too many time-gated events
I just don't see how this is a bad thing. I think being rewarded for playing the game at certain times is super cool, and it helps the world feel more alive.
>Evolution stones are unnecessary hard to obtain
This is the one common criticism I see of HGSS that I can kind of understand. Personally, I never really had an issue with it; I find it really rewarding to do the bug catching contests and Pokeathlon and then such, but I understand that it isn't for everyone.
Anyway, why exactly is this game hated nowadays? It's not perfect, but I personally think it's the closest we've ever gotten to a perfect Pokemon game, and I've played them all (except BDSP and SV).