We use flask at fusic.com and find it very reliable and easy to work with. Our peaks have over 1m rpm, and it has never fails us. We did a huge project migrating from Django, and now we don't have to jump through hoops so much.

I think flask has a front page that discourages newcomers, as it does not look as serious as Django, and gives the feeling that is it not mature enough, or documented. I tried convincing a new startup to use it and failed for those reasons.