Checking my monitoring logs I have 26 minutes of downtime attributable to Netlify since 03 March 2017. That's about 99.994% uptime.
If I attribute every single outage. That is 43 minutes since 03 March 2017, though some of those were not Netlify errors, but even so, if we are uncharitable/inaccurate and attribute them to Netlify that works out to 99.98% uptime.
Not bad for a free service.
They're also very transparent about service issues:
And support is very responsive. I recently talked to them about deprecating TLS 1.0 and 1.1 for instance, or providing the option to force TLS 1.2 if users desired. They were quick to respond and helpful.
So if you are seeing unusual issues, get in touch with them, even if you're a free user they'll still talk to you.