For transparency, I'm not very well versed on this topic. That said, one concern I've always had with flywheel systems is that it ends up being a crazy heavy thing spinning at a decent rate. I don't know how valid it is but many years of being around maker communities and stuff has given me a respect for heavy things spinning at decent speeds.

They do seem like they would be a simple effective way to get some brownout or power flicker protection though!