I struggle to outright call Sonic Forces a "bad" game so easily as many do. However, I wouldn't call it "good" either. I think the best phrase to describe it is "Disappointing and painfully mediocre".
The story is half baked with not only a massive amount of missed potential, but with characters regularly acting one dimensional and/or out of character. It feels like the writers barely even knew who these characters are or how to use them in the story. Classic Sonic in particular does little to nothing of note in the story with everything more happening to/around him than because of him. I could go more in depth, but others have put it better than I could all over the internet already, and I don't want this post to be any longer than it needs to be.
The physics are a bit jank. It's very difficult/tedious to do precise platforming as Modern Sonic or Classic Sonic, and things are only marginally better in this regard as the Avatar. The Wispons are very fun to use at least, so there is that. There's less control over the player character in 3D than in Generations however. Drifting for example was removed in favor of automated sections for the Avatar, with the Avatar being required to drift at all. Even Colors allowed Sonic to drift in certain sections at the cost of being unable to boost, Forces doesn't even allow that much.
Character customization doesn't allow you to make characters that look like legit Sonic characters, they look more like inferior bootlegs most of the time. The customization itself isn't as in-depth as I would like either, but I did have fun toying around with the creator and unlocking new clothes to equip. I'd actually like to see an improved version of this appear in future titles, perhaps in a spinoff series of some sort. I actually really like the idea of smaller side adventures featuring the Avatar, perhaps with local and online coop of some sort.
The Shadow DLC is pretty mediocre. The best thing to come of it is the new remixes of existing music tracks associated with Shadow such as the Westopolis theme. The DLC only has three levels based on existing stages, and nothing too special in regards to story content. Completing it does allow you to play as Shadow in select Modern Sonic stages in the main campaign, but he only functions as a skin with no actual difference to gameplay and no additional story content. It's a free DLC though, so for the price it's asking for I can't really complain too much.
Overall Forces is "fine" in that it does have some enjoyment value and redeeming aspects, but it's one of the weaker entries into the franchise by a pretty decent margin. I expect that as time continues to pass, Forces won't even live up to the same level of infamy as Sonic 06, but rather is more likely to largely go forgotten by all but a select few who actually hold it in high regard.