They are based in the US and only provide their services to radical leftist activists.
That combination seriously smells like FBI honeypot.
They're pretty chill on who to provider their services to. Yes, they host radical leftist activists, but they also host pretty mainstream leftist activists. And they obviously don't care who does what, they don't check your accounts.
I'm far from being a radical leftist (or any leftist!) and I have an account.
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.)
I have been a riseup user for years. They have received foreign legal requests, and they do not simply do the task requested. They've also received US-based legal requests, and challenge them, but in the end, they do not have the data that is being requested, so ultimately they can respond saying exactly that.
(throwaway aswell)