The single-player stages are a chore. The first ones are incredibly basic and bland, and the later ones are either stuffed with too many gimmicks or feel boring to play. The game is incredibly easy, even as Sonic in Ultimate mode, so there really isn't any challenge to anything so I'd never have to use much effort to play.
Aside from the occasional Match game, the multiplayer sucks. Coop isn't cooperative, Race is a bland Thokfest, CTF is monotonous and repetitive, and Hide and Seek is just plain awful, as all everyone does is hide in corners as Knuckles, and the map selections are all poor to begin with. Match isn't too fun, though it does have its moments, at times.
The community generally sucks, with whiny little kids all over the place, hosting WAD-infested servers, roleplaying, or they're too stupid to actually play the game properly. There's a ton of annoying people, and most of the old people of the commnity are elitist. There's only a small handful of people that I actually like, such as ICE and Jellybones.
The graphics of the game never impressed me. It's 3D, but I dislike the style, everything looks pixelated, and there's a ton of graphical errors and limitations. And, to make things worse, it's very difficult to actually hit something (especially the "final boss"), because you can't visually see the hitbox of 2D sprites. There's fairly little in the way of graphical effects, and the menus seem boring.
I dislike just about all of the sound effects and music tracks. I have to turn the volume completely off sometimes, and listen to a track on YouTube or something. The only songs that I actually do like are Techno Hill 2's song, Spring Hill's song, Sunken Plant's song, and the original version of Midnight Freeze's song. I really can't stand any of the other music, especially the tracks used in Egg Rock, Arid Canyon, and Greenflower Zone.
The game's incredible age really shows. There's a ton of glitches, limitations, and errors with the engine. There's a vast amount of things that we can't even use, and those are things that could've made the game actually good. The graphics sucks, we can't do many complex features, and the netcode is just awful.
You can customize things, but there really isn't much to it. Character editing is extremely limited now, and it's very monotonous and the new character probably won't even make a difference. Adding sound effects and music is easy, but it ultimately isn't worth it most of the time. Replacing graphics is also easy, but there's annoying palette limitations. Making levels (which is the sole reason I actually have the damn game) can be really monotonous and it takes a long time to actually achieve some progress. SOC files let you make some actually innovative things, but it usually ends up breaking other things or the end product sucks. The best way to modify the game is easily by coding, but even that has limits.
The content released by the community usually sucks. The levels being downright awful is commonplace, but all the idiots praise it mindlessly and the person continues making the same stupid flaws again and again. Character WADs are almost never finished, and they're mostly recolors, edits, or badly-formed sprites.
tl;dr version: srb2 is vastly overrated, as most of the features downright suck