But change a working virtualized system from BIOS to EFI and there is a good chance a bunch of other things will break. Lots of memory mappings and hardware detection stuff changes if you switch from BIOS to EFI, together with the likely need to repartition or add a virtual disk for the EFI boot partition.

As far as I can see, only Hyper-V enables it by default as of today; this means most of the VMs being created _right now_ we can assume to be BIOS rather than UEFI.

> libvirt/kvm supports it,

I recall reading somewhere that using UEFI on qemu/kvm caused problems with snapshotting and/or migration; is that still the case?

libvirt/kvm sort of supports it, unless you want to use snapshots I guess. https://bugzilla.redhat.com/show_bug.cgi?id=1881850

That bug appears to be a duplicate, I use legacy bios type for all my VMs to get around this since it's been a problem for at least 5 or more years.