I wouldn't call it "writing on the wall," but they have done some not-so-good things over the last few years:

1. Handing over data for their email services (which was legally required) (ref).
2. Releasing a Bitcoin wallet. The problem for me is that Bitcoin is inherently not private.
3. Lying in marketing. Proton claims "no data or speed limits" for their free VPN (ref), which is just plain wrong. If you download a few gigs, it will slow you down to a few Mbit (if I remember correctly). I even contacted their support about this, and they just said, "They are balancing the servers for the free VPN." But then why was it fast in the beginning, and if I reconnected to the same server, would it be fast again. Just to be clear: I have no problem with the speed limit/balancing itself, just that they are lying about it.
4. Proton incentivizing free email accounts to connect to a Gmail account to get 500 MB more storage. (You need to go through the "tutorial" steps to get the 500 MB extra, and one of them is to have a Google Mail account send all their emails to your new Proton inbox.)

This is why I personally decided against Proton.