1. I can't write an email from an alias. For example I can't send an email from something+yourservice@example.pro. This is possible from Thunderbird using their bridge, but not from their email app on Android. I can reply to an email and it will preserve the alias, but I can't write one from scratch.
2. Emails are not downloaded locally on Android so I can't access them when internet is not available.
These 2 have been consistent issues for me over the 3 years I've used them, and when I've contacted their customer support about it I was told they have no intention of fixing them.
I use Proton on Android and I can write a new email using one of my aliases. Maybe it was fixed recently.
Regarding n° 2, I think it will never be fixed, given the nature of the service itself, in which all emails are stored encrypted, and the encryption keys are in the server.