It's true that career risk is low, but opportunity cost could be high. If you're well into your career, taking a few years off to work at a startup that might fail could really be a million dollar tradeoff.
So you really gotta believe in your startup.
(Note: I left my comfortable high paying job earlier this year to start a startup)
At the time I was pretty burnt out from teaching, ended up moving to a new city and was doing a few temp jobs to make some extra cash. The risk was low enough at that point that the website was almost a no brainer. I'm pretty confident that had I been in a good spot career-wise at the time, I would likely have either never kept my side project going.