That's the only reason I won't use Proton, because the app and the network is not clean. The best security encryption is made from public protocols, so app and servers should be open source.
Servers can not be made free and open source, that is just not how it works. There is no way of proving that a server can be trusted if you do not control the server.
I did open an account once (for the free vpn), but ditched it very soon because it could not use a normal mail client. I need all my mail in one place, and I need convenient PGP however rarely it does happen.