Three things.

• There was a legal situation years back and the Swiss Corporation was approached over the contents they held for a customer in France(IIRC) and some of the French people reported that when they complied that Proton had given them Everything they had on him, which wasn’t true, all they gave was the metadata that was in the clear already. Some of the news sources, including the first few I saw on it, had the French framing and reported it as such. A privacy company just giving up a users data like that left a bad taste in peoples mouths, and it turns out only some of the original reports corrected the false narrative and just pulled the story down. I had to personally eat crow on that one with more than a few people.
• A founder and C-Suite member of the company personally supported the Trump Administration several times allegedly without discussing it with his more centrist and left leaning collogues. Proton as an organization did not yeet him out over that, but recent ICE and military activity did not change his mind.
• Proton, and many other privacy first services are offering common services for free but with limits that no long term Google, Yahoo, or Apple Mail/Outlook user can operate in, let alone for all their services, and their first paid tier is also too anemic for shared storage, so you are stuck either deleting your content or paying for their highest paid package. This for apps that are threadbare as instead of flushing out the featureset they have allegedly focused on putting out new products more often. This isn’t that popular.