QEMU is more well-known and tested than Firecracker; i.e., a hacked version is used in Xen used everywhere in the past decade while Firecracker is primarily an Amazon-only thing. Cloud Hypervisor, Dragonball, and StratoVirt aren't well-known or battle-tested IMO. The problem is none of these possess true manageability and isolation features of any solid type 1 hypervisor which makes Kata equivalent to a user-space application rather than a reliable platform with harder resource isolation guarantees.
Firecracker is probably the 2nd or 3rd most widely deployed hypervisor in production deployments. I think "Amazon-only" isn't doing the rhetorical lifting you mean it to do. The idea that it's "equivalent to a user-space application" makes very little sense.
No, that's true, VMs don't protect against microarchitectural attacks. But neither does shared-kernel isolation; in fact, shared-kernel is even worse at it. So if that's the concern, it doesn't make much sense in the threat model.
