The great thing about no trust models is that even if they were comprimised it wouldnt matter
Unless of course you were using PGP - but then you would be about as well off with Gmail?
I mean, they provide a great service, and the stack is open - but I would hesitate to call it "secure" (that goes for ProtonMail too BTW).
If riseup were a trap, they could do all the logging (in secret), find the activist, then do parallel construction for the actual evidence presented in court.
Before you say, "Well, it could be done in secret", please look up the Wiretap + technical assist laws and note the disclosure requirements.
(There may be some solid evidence making this unlikely in reality, but that ultimately relies on some form of trust.)