I can't comment on the Nabla parts as I don't work on that and have not read the code in any detail, but upstream Solo5 (and thus MirageOS on Solo5) has gradually been getting fixes for most of the "security posture" issues identified by NCC in this paper over the last few months. Note that not all of these issues apply to Solo5 in the first place.

The notable thing missing at the moment is ASLR and a more robust guest memory layout in general. We have an issue with a plan for that but it's a matter of balancing a finite amount of time/manpower and priorities.