the thing is, Sonic Forces on it's own isn't that bad of a game...
But it's painfully mediocre.
The basic promise of "Eggman win, now Sonic & co. have to defeat him in a world he control with a iron fist" isn't a premise that's never been done before, if we count the comic/cartoon show, but it still had a ton of potential for the game canon.
Like they all have those cool ideas that, whatever they make sense or not, could at least make for some great setpieces and/or show us those characters we loves in way we haven't seen before.
But NOPE, every time something interesting come up, the game goes the least interesting & safe way possible to go about it and in the end, it just feel like alot of nothing. (See Null Space being skipped in the first 10 seconds of the level despite a cutscene building up to it, or Eggman conquest of the world being not show but instead told to us with whites text on black background)
There is other thing worth mentionning I guess, but really, for a series that is know for being at least entertaining whatever it fail to live up to expectations, Sonic Force being so boring really make it worst than other titles in the franchise in a way.