I use Proton mail for a mailing list that’s hosted and managed by a local linux users group. The messages from the mailing list arrive as .eml files, with each message as an attachment. the native web browser cannot read the attachments. I have to download each message, either individually or all of them as a single zipped file. It might be the fault of the admin of the mailing list and not Proton’s fault. I’m not sure. It’s not very active so I never bothered to look into the issue. it’s a hassle but not a problem. I thought .eml was a standard email format so it seemed odd that the web client could not read it.
i also occasionally use proton drive to back up my plaintext journal every 3-6 months. i backup to mega as well. proton drive has 2 gb of storage on the free plan. mega has 20gb. my journal is 6.9 MiB across 166 files. i have plenty of storage for my use case. i do not store anything sensitive. so that’s not a concern.
This was first published in 2021. There are some interesting points made.
https://dessalines.github.io/essays/why_not_signal.html
It has had a few updates since, then but I cannot vouch for its accuracy.
It doesn’t cover audits per sé, but I feel there is important information that is tangentially related, since security audits become kind of moot if some of the items mentioned are true (i.e. CIA funding and US govt. tactics).
Full disclosure, I still use Signal for a family group chat. I have very little economic value, thus my threat model is minimal. It mentions cats several times. I neither have cats, nor interact with them frequently enough to warrant their inclusion in a threat model.