I say go for it!
Can't claim great experience - the startups I've been in never went far. But where the other commenters seem to be implictly a bit skeptical, I think the risk/reward ratio is good enough in the circumstances that you describe that the decision is in the personal zone.
And you sound like you want to try it; you've "had to pass up" other opportunities because they were unpaid/unfunded, etc. You describe your current job as boring, state you can find other 9-5 jobs, etc.
I agree with the other posters that finding a solid 9-5 with great commute and safe, reliable, already-working path to FI is not something to drop lightly. However, based on your comments, risk is limited because you are fairly comfortable in your belief that you can return to the safe path.
The experience factor is hard to measure, and in my opinion someone who doesn't supervise anyone in their day job even though they're experienced already in their field won't magically become substantially more valuable in non-startup situations just because they went through a brief startup adventure that failed despite funding. So IMHO the key factor is reliability of finding other jobs next year if this company doesn't become sustainable.
That said, even if the risks are slightly greater than the quantifiable rewards, your comments imply that they are manageable in size. So if you want to do it, you can. I say you should!
In my view, this is a gut question. You've already stated this is a rare opportunity of low and manageable risk. Effectively, you will pay to play: a year or two of delay in FI versus a chance to adventure much earlier. The upside is then bonus. Go for it and good luck.
One person's opinion, anyway.