"Of similar maturity"? What does that mean? What is one mainstream application --- let's define "mainstream" here as having more than 10,000 active users --- that is built directly on top of the Urbit stack?

By maturity I meant something like "continuous opeartion for a year with cryptographically signed live update (including cryptography themselves, implemented in VM)", which Urbit did. But okay, I am also interested in competing projects of any maturity.

Because implementing cryptography in a bespoke VM is an important feature? Why? That's the kind of thing cryptography engineers try to avoid.

You haven't named even one competing project. I think it is a good justification for interest in Urbit.