Easily my favorite map pack in the game. I've always thought this map pack was really in line with vanilla SRB2's design philosophy (and I love vanilla SRB2), which is funny considering it handily predates 2.2. Maybe it's more accurate to say 2.2 is in line with Tortured planet's design philosophy, but I'm not an SRB2 historian. Started with 2.2 and that's generally besides the point anyway.
I don't really have any strong opinions about the aesthetics. It's not an ugly pack by any means but it's nothing to write home about. There are a few objects made with sectors and FOFs that really shouldn't be, but I think they're funny. I don't really care too much either way, because the level design is what I love about this pack.
I really like vanilla (2.2) srb2, and I don't tend to play with addons because they just speak a different language. I have lots of thoughts about addon character design lol (not that they're bad I just don't like playing with most of them). But Tortured Planet just vibes in a way other map packs don't. It feels more like an extension as opposed to a different beast entirely like something like the 06 pack for example (again not that 06 is bad I actually like it but vanilla is still my GOAT)
I don't find it's level design any more confusing than vanilla, though maybe that says more about vanilla SRB2 than anything. I do very much find it harder than vanilla, though I don't think that in and of itself is a valid criticism. It is hard yes, but I don't think it's unfair really and I never ended up getting too pissed with the pack. There's not enemy spam or anything it's just sincerely difficult platforming challenges (though there are some deathplanes that should really just be damaging floors *cough* fume shaft and alien armageddon *cough*) And I understand that for some, maybe most, the level of difficulty this pack provides is just not fun, but for me, I really enjoyed it, and I don't think you can judge it just for being something you're not predisposed to enjoying.
As a note, my first playthrough was with Fang (because he's my favorite character in the game, including addon characters don't @ me lol), who is one of the easier characters, and my second was with Amy, one of the harder characters. My perceived difficulty of and experience with the pack might be skewed by a first playthrough with Fang (and also by being an epic gamer B)), but it's not a secret that Sonic's the hardest character in the game, so it's kinda on you if he was your choice. That being said, it is a bit of a secret how hard this pack is, so I can't blame you too much. Funny enough, this pack predates both Fang and Amy. The level design gels just fine with them (I mean Amy really isn't enough of a character for there to be anything to conflict with), but there is a softlock in drowned downtown for nospin characters, and probably in other places too.
Aaaaaall that said, it's not free from criticism. The bosses are massively nothing. I don't expect much from bosses in map packs really (I mean I play Fang so I don't think much about bosses at all), but it is true that the bosses in this pack kinda suck. Doesn't even use all of the vanilla ones and ends up reusing some IIRC. That might be a produce of its age but I'm not sure. Using old brak is definitely a product of it's age and also makes me very sad because I do not like old brak (especially as a nospin character).
The last acts are also exceptionally long. I occasionally see SRB2 speedrunners do alien armageddon 2 in particular because of how massively long it is, which is kinda cool from that perspective, but from the perspective of an actual game you play kinda sucks. Liftoff gantry zone specifically felt like a real slog to me.
Also also spacewalk zone. It's my least favorite zone. It's just weird. The pack as a whole can feel jank at times but spacewalk especially. It's sense of scale also gets out of whack sometimes, which is something the rest of the pack is rather good with. IDK I never had a lot of love for ERZ and its assorted gravity gimmicks so maybe it's just a bad match.
Anyway it's a good pack. Maybe it only deserves 4 stars because it's not completely perfect but if my favorite addon doesn't deserve 5 stars then what does?
EDIT: Brak is dead! Rejoice! I went back to check that this pack actually used old brak because I thought I might've been confusing it with something else. I wasn't, but they got rid of old brak anyway! Also the drowned downtown softlock I mentioned is fixed according to patch notes, though I didn't check in game.