I have done in-depth research on this company, it has a great vision. However, I have heard or read about issues in the integrity of the founder. This is my only opportunity to become a co-founder, as the company is the only one of its kind in my area, in which my skills match the needs of the company.
The biggest question you'll have to answer at this point is whether you and the founder can get along. You've already highlighted issues with the founder but does this bother you? Are both your goals aligned and do your values agree with him/her?
In a previous experience, I brought someone on whom I didn't know much about - only that his skills matched what I was looking for. However, only after a few months, he decided to leave as he didn't agree with the new direction we were taking. Yet, there was never any opposition when the idea was first brought up.
The team is one of the most critical things at the beginning of every startup. You'll want to be in an environment where everyone is open, honest and can execute to the best of their ability.
I don't believe that this is your only opportunity - there are plenty of opportunities all around to those who seek it.