Still playing this. I left detailed thoughts awhile back, so won't go too in-depth here, but I'm up to the third island, so here's some general stuff:
-I still hate the Cyber Space stages. Some of them are actually pretty decent, but "some" is the key word. I've just played two that were taken out of Sonic Unleashed, except they're worse in every regard. I'm actually going to revise my statement and say that they might not be worse than Sonic Forces, in that the good ones are better at providing alternate routes, and the ranking system is better, but in terms of actual FUN? Yeah, Forces might still be better.
-The open-zone areas are more fun. There's a fair bit of jank at times, but you can also use the jank to get around if you know what you're doing. And the combat's still fun. On the other hand, if you're asking me how well Sonic controls in 3D, I've gotta say that SA1 still holds the top spot after all these years.
-On story stuff...I'm going to try and minimize spoilers here, but anyone who knows me knows that I'm more interested in analyzing story than gameplay (and the same goes for wikis), so on that note, having reached the third island, I'll say at this point that while I don't think Frontiers is the best Sonic story ever written, I think it might be the most mature, rivalled only by a handful of entries. And thankfully, it's a sound type of maturity, not the false, "edgy" maturity that entries like Shadow the Hedgehog and Sonic Forces tried to pull off. It doesn't always stick the landing, but here's general thoughts on the matter:
a) If I had to sum up the core theme/motif of Frontiers in one word, it would be "death." A lot of you might scoff at reading that, but I'm serious. Death, legacy, melencholia, all of these things are practically omniprescent everywhere in the game. There's a kind of inherent sadness to the entire game - again, I won't divulge why that is - but based on everything that's been seen and revealed so far, Frontiers is a game where the weight of its environment and its history is constantly bearing down on you. Cheesy as this sounds, when it starts gently raining, coupled with the ambient music...yeah. I'm sure you can point to any number of games where this is also true, but in this specific context, it works.
b) By extension of point a, there's the environmental storytelling. I touched on this last time, and having reached Island 3, this holds true. Island 1 (Kronos) is relatively verdant, even if you're dealing with ruins (and there's the "tombstone reveal" I mentioned last time). Island 2 (Ares) is desert, bar a single oasis. Considering that you shouldn't expect islands this close to have different biomes, yet you know what happened here long ago, you can probably suss out why Ares and Kronos are so different. Then, island 3 (Chaos), you have a barren landscape - not desert, as in, completely barren - magma instead of water, for instance, far more military equipment, etc. Compared with what we see, it's kind of stark how well the names alone fit - Kronos (god of the harvest, verdant), Ares (god of war, more damaged, signs of war machines, more advanced enemies), and Chaos (in-universe "God of Destruction," island has been completely devastated).
This is going to sound silly, but there's one environmental piece that really got to me here, and that's a destroyed Death Egg robot - the mass produced type seen in Sonic Forces. Over there, those things were hyped up as being unstoppable, and while you did see ruined robots in the background, there was little actual impact. Here, you can climb the thing, and it's striking how one of Eggman's war machines just...fits the environment - one weapon of war, amongst numerous weapons of war, on an island that's been scorced free of all life (which we see in a flashback as well). I'm almost certainly reading too much into this, but hey, feelings are feelings.
c) I'm returning to point a by doing point c (work it out, scrubs), but there's the storytelling/character elements as well. It doesn't hit all the time, there's a few cheesy lines, and the pacing doesn't always work (because many conversations are optional, these can jank with the flow of the main story, which is actually pretty threadbare), but most of the time, it does. I've already commented on Amy's themes on Island 1, Knuckles here...he's handled well, but again, the theme of death, loss, regret, etc. weighs down on the characters themselves, and by extension, the koco. I won't spoil what the koco really are (though you can work it out long before the actual reveal happens if you're paying attention), but basically, their actions and desires act as mirrors to the characters so far. For Amy, love, for Knuckles, loss. The game arguably even recontextualizes Tikal from SA1 - I won't give the line since it's a potential spoiler, but basically, based on my reading, there's the implication that she was able to endure as a spirit for so long because of grief, and by extension, the idea that Sonic and co. helped her find peace to the extent that she was able to move on. Again, might be reading far too much into this, but the events from past games are dropped ad nauseum (Flynn knows his stuff), so it wouldn't really surprise me.
So, yeah. I don't know if I can call Frontiers a "great" story, and arguably, I'm more positive to it than it might deserve from the fact that for the past 10 years plus, Sonic games have either not given a damn about story (starting from Colours), or in the case of Forces, failed spectacuarly. But even then, it's actually kind of startling that this game is able to convey the emotions and themes it does, and not feel hackneyed in the process, said series having started with the premise of "evil doctor is kidnapping animals, hit him eight times to save the day." If you want to know why I still keep coming back to this IP after it giving me countless reasons to just move on, Frontiers is part of the reason why.
Or, if you want the TL, DR version, Cyber Space sucks, open-world is pretty fun, story is pretty neat.