https://protonmail.com/blog/transparency-report/
On the other hand, they recently reduced the level of detail in the transparency report.
There is also the fact that they are Swiss, and their privacy laws were severely weakened by a recent referendum. In particular, the Swiss government can now monitor all cross border traffic without a warrant.
ProtonMail fought the referendum, but hasn't updated this "Why Switzerland?" page:
https://protonmail.com/blog/switzerland/
They also haven't moved to a more appropriate legal jurisdiction.
[edit: clarify links]