Going in without polish is going to be hard. That being said I did exactly that. When the first Sword & Laser hit I was planning my book and about a month out from writing. I quickly whipped something up and jumped in. It was stressful, don’t expect to get much writing done when you are campaigning.
Nothing like a trial by fire to vet an idea, and as an added benefit submitting a book to Inkshares is very low risk. If your funding is not successful it returns to draft state and you can attempt to fund it again at a later date when you feel ready. You’ll retain the book’s followers and any pre-orders will be refunded. Aside from the stress associated trying to fund a book (which is completely up to you,) there is little reason not to throw a book in the mix.