For me it's their ransomware-like business model of: attracting new users with free accounts, incentivizing them to migrate their Gmail inbox to Proton (Easy Switch), encrypting the content ("for security"), and then letting them find out that a paid feature (Bridge) is necessary to integrate the mailbox into a third-party client like Thunderbird (as a decryption layer in between); well, unless you want to manually export the mailbox using their Mail Export Tool. I'm not opposed to paying for a service (I also donate to Disroot), but I dropped them like a hot brick when I found out: thankfully not the hard way (I tested integration before migration).