The desktop client is basically a web app, little different from running it in the browser. As for the app, I have been pretty unimpressed by it; performance is sluggish, and emails are displayed poorly (lots of unreadably small fonts with no options for text resizing). Worst of all, development seems to be either inactive, or so slow as to be undetectable.

I paid for Tutanota and started switching to it from Gmail, but the accumulation of inconveniences is starting to make me consider switching back, in spite of all the Google privacy issues.

IzzyOnDroid F-Droid Repo has Protonmail. Proton has a mail bridge application which let's you use desktop mail clients.