I think it had potential to be a very decent Sonic Adventure 3, but it's ruined by a lot of issues.
I don't think it's a terrible game in itself, I actually enjoyed it, but is the overall experience great? No, because of its various problems.
One of the worst problems in my opinion are the loadings, they are HORRIBLE, they're extremely long and when doing side quests you will always have the loading twice, because for some reason they decide to reload the whole scene just for one line of dialogue from the NPC you're talking to, and then reloading the scene for the actual mission, it's extremely stupid and I can't comprehend how they thought this was good.
It also has frequent bugs, collision issues and various problems making the gameplay less enjoyable than it could be.
It just feels like a beta of a cancelled game or something, it's unpolished, unfinished.
I do think however that it could have been a decent game if they decided to port it to new consoles / PC with fixes for all the issues people mention all the time, and maybe some reworked visuals since it's been a while now and some areas could use improved lighting and such to make them feel less flat. If this ever happened (which I know it won't) I would be ready to play it again if it was in good conditions.