I looked at what they are offering. They offer basic web and mail services. An app for social networking.
Saying that they support Feminism doesn't mean much anymore, but I searched for "LGBT" and didn't find it. So that's a good sign.
it had a very Web 2.0 vintage vibe to it. Perhaps this is where we need to go to host our communications, but returning to that level of tech gives me a stomach ache.
I don't know if I know anyone who's signed up for their email services, but I have used the riseup pad as a perfectly serviceable privacy-friendly substitute for a collaborative google document in the past!
I would caution against openly using the term radical feminist or "terf" if you do correspond with them, but as far as I know they are legitimate.