Proton has a very shady past (mostly in the mail server area, their initial business, easy to search for details as they all were reported reasonably well). I always try to get people to pick literally any of the other alternatives, and it's also why I went with mailbox for my own mail when leaving Google.
Being in Switzerland means they are also not in the EU and not subject to many of the customer protections you would get if they were.
A relatively recent example, basically just one of the first search results.
Then there was the CEO endorsing Trump, or his politics, or one of the controversial nominees. It's been a while so I don't remember the exact specifics, but at the very least it caused quite a stir and caused backpedaling by the company (and the fact that it was from his personal x-account, not from the company).