I'm a customer for more than a year and never had any noticeable outages-issues so far. A few times, I encountered issues in their UI, but I'm also using the beta-version so that's to be expected.

FYI, there's also an awesome unofficial open-source Electron client for ProtonMail, ElectronMail: https://github.com/vladimiry/ElectronMail I prefer a standalone client but still like their GUI over thunderbird et al; ElectronMail fills this gap perfectly