Ask HN: Do you use Soverin MX, is it reliable?

A recent post[]1 on a user moving from GMail to MailBox sparked a lot of discussion etc. There's an email service which has been around for a decade which has had all of 4 no-traction posts on HN over those same years which I've kinda thought was strange, "why don't they come up organically in HN MX-oriented conversations?":

https://soverin.com/

Has anyone actually used this for their custom domain and can share commentary on experience (admin, user, reliability, etc.) compared to the other popular choices? (Fastmail, etc. - "MX ISPs" if you will). One key feature for me is being able to have app passwords for different tasks/software allowing one to limit read/write access (they have scope, which many providers don't do but Fastmail does).

This is a backup mail provider oriented question, not leaving Fastmail but I maintain a simple secondary domain at another MX ISP for emergency purposes and archiving. I am investigating a move to a new MX ISP at the end of the year when the current contract expires and Soverin keeps "looking good" - is it in your opinion?

[1] https://news.ycombinator.com/item?id=44987380