Sonic's problem is that it changes too quickly and it takes itself too seriously. I mean, if you look at the past several games, they've gone through about ten different main game mechanics without sticking to one. That, and this game in particular is playing itself off as one of the most gripping tales to ever be told. The line "I'm an optimist, but I'm also a realist; Sonic is gone for good." is a line that actually exists and is real. It's a Sonic game. The story is that he slams into robots and saves enslaved creatures. That, and he goes fast. The end.
A Sonic game can take its story seriously and still be good. Where it runs into problems is when the lines are so damn terrible that it makes long-time fans immediately want to turn the game off and get a refund.
And Forces in particular has a problem of things happening for no reason at all. Like, sure, the reason why Classic Sonic was there was because of the portal at the end of Mania, but after he gets there he does nothing of importance, other than being a selling point on the box.
Not to mention only half of the enemies from the reveal trailer even get boss battles. Shadow just enters a cutscene to explain how the Ruby works, and Chaos does
literally loving nothing before getting ohko'd by stuffty Sonic 4 physics Sonic. We only get to fight
yet another metal sonic and the unimportant red guy from Lost World. What was the point of the other two being there?