The question is super subjective. It's entirely possible that someone's opinion could be Sonic Adventure is a bad game while absolutely loving Forces, it really all comes down to personal tastes and standards. Personally, I don't even consider Forces a bad game, just painfully mediocre and disappointingly missing it's potential. The Avatar stages are fun to me, Classic Sonic... Exists, and Modern Sonic's physics aren't quite right and his stages are too linear without much in the way of replay value compared to Generations and Unleashed. The story... Is a hot mess that I'd rather not get into, as I'd end up ranting about it for multiple paragraphs and it's all a digress from the main point of the thread.
Overall, I'm not worried at all Frontiers might turn out bad, the gameplay we've got so far as well as the reactions of those who've got to try it have reassured me that it will be fine. I'm a lot more worried that the full game might end up being another mediocre title. Even if it ends up being more good than bad, if it ends up being yet another mediocre title, it could end up being horrible for the franchise as a whole. The game itself as a standalone product would probably generate it's own fanbase in time, but the franchise is overall starting to get viewed as damaged goods run by incompetent/uncaring producers. It could have a Sonic 06 effect on the fanbase if it's not an absolute home run in that the fanbase will get fractured yet again. That's my primary worry. This game can't just be "decent" or "okay", it needs to be "fantastic" and "iconic". Anything less and I fear that a lot of people will just give up on the franchise, especially with so many people on edge after Origins was just "okay" among it's various issues.