For me it was freezing frequently when downloading emails, and became unusable over time. FairEmail was a bit better, but the UI is awful unfortunately. In the end I've ended up with the Google Mail app - not my first choice but it works well with multiple accounts and is easy to use.
To me Google Inbox was the best incarnation of an email client ever (both the mobile and web app). They have quite inexplicably pulled the plug on it though.
To me it was clear on the day when they pulled the plug and dumped everyone back into regular gmail: this was a product that was worked so well, that you no longer saw things that you were not going to interact with and were not wasting your time in this attention economy. And that was a “bees against honey” situation and it not allowed to continue.
People who created Inbox they should be proud of themselves!
