If you really don't believe in it and you don't think it'll go anywhere, if they raise a huge snit and you don't want to deal with drama, you could even offer to give back some shares if any have vested (if they are going to be worth 0 anyway) or you keep them because hey, you worked for them!
They don't need to know why you aren't interested or that you think it will fail. If you don't feel comfortable telling them this already, then there is probably a reason for that when it comes to their personalities. If something isn't a fit for you, don't waste your time and energy on it. If you've got these skills, you know you can go elsewhere. Better to leave early then run something that barely works until it fails. Fail fast and learn from it. Maybe other's don't learn as fast, but that is their own fault.
Good luck dude.