I suppose it comes down to a few basic things...
Level design is the most significant issue here. Having just spent the afternoon replaying parts of Generations and Unleashed, in order to compare and contrast, the main difference stems from minute to minute complexity. It's not that those games are "complex', per se, rather the level design rewards and encourages skilled play. There are multiple tiers in each stage (not unlike 2D Sonic but narrower in a sense) and it's easy to fall down to the slower path. The "top" path is designed to reward those that can react and deal with challenges quickly. You'll be quickly dodging, jumping, ducking, boosting and interacting throughout each stages and doing it well feels fantastic. I always got an emotional high from playing Boost Sonic well - the combination of music, speed and skill help create something memorable.
Sonic Forces, in comparison, features levels which are roughly 50-75% shorter on average requiring just over a minute to complete in many cases. Each level consists of a very straight path with no deviation, lots of scripted sections and little challenge. It truly has become boost to win in that sense. Beyond that, when you do slowdown, the game feels very blocky as if the team simply connected a bunch of square objects together and tossed a few enemies in. It feels cheap and poorly made. You go back to some of the best Unleashed and Generations stages and they feel CRAFTED by a talented designer - lots of bespoke level objects and segments. Forces feels like a mix of straight, barely interactive boost tracks and blocky, uninspired platforming.
Then there is the pacing.
The game is driven by an overworld map presented as a globe. On this globe are level scattered all about while pressing Y or triangle brings up a list of possible stages. You see this map a lot. The basic flow involves selecting a level -> listening to a dialog sequence presented using text boxes -> watching a cut-scene (often) -> playing the stage -> looking at the victory screen as it rewards you with tons of unlockable clothing for your avatar -> going back to the map.
Let's say it takes 3 minutes to go from clicking on a stage to returning to the map. 1:20 of that might be the stage but the rest is taken up by dialog, cut-scenes and reward screens. You spend more time looking at this stuff than actually playing. So even though the levels are short and simple, they can still feel "fun" at times, no doubt, and the music is awesome. It just feels like they're giving you a small taste and, just as you get into it, the stage ends.
This is made worse by the fact that the story, its pacing and the tone are just so awful. Sonic has done serious before, that's fine, I could enjoy what Sonic Adventure 2 was selling. I could enjoy a "serious" Sonic story. The issue here is that it's presented in random chunks which don't feel connected. Like, you'll play a classic Sonic level and, after finishing it, the game shows a cut-scene taking place in a completely different location with different characters set in the past that has nothing to do with what you were just doing. Each cut-scene is short and written in a very terse fashion so it never feels like it fills anything in.
The game just loves to bounce around between random scenes and levels that aren't well directed without any cohesion.
So there's a very start/stop feel to its gameplay since levels are so short and the surrounding package feels so bloated and the story telling is just all over the place. It ruins it for me in a big way.
What's worse is that they focus very heavily on customizing your avatar, completing daily missions (ie - equip a new pair of gloves to double your EXP for 40 minutes!!!), and replaying levels. It honestly feels like a shell built for a mobile phone game more than anything else.
All of this combines to create something that just keeps making me feel worse every single time I play it. There are moments of fun in here where you get a feeling not unlike Sonic Generations but they are so few and far between. THAT is ultimately what has ruined the game for me.
