What's wrong with internal VM snapshots?

Hi! I am trying to improve the support for VM snapshots in the Cockpit web console, and I am afraid I have questions... We have been asked to prefer the "external" over the "internal" snapshot format, at least on RHEL. I haven't yet figured out why, and consequently I am struggling with deciding how hard the Cockpit UI should push people towards external snapshots. So, what's wrong with internal snapshots? I heard they are "unreliable", but how so in detail? Does the data structure inside the qcow2 files get corrupted easily? Do they behave poorly when the snapshot process runs out of disk in the middle? That sort of thing would help me a lot to figure out what Cockpit should be doing on platforms other than RHEL. And how well (or how soon) can external snapshots be expected to work? I have severely messed up my libvirt state a couple of times while playing around with them, and my confidence in them right now isn't great. :-) Are you surprised by this? Or are external snapshots not yet considered ready? (Most recent example from my experiments: Deleting a full system snapshot of a paused machine fails with "internal error: unable to execute QEMU command 'block-commit': Block node is read-only". Reverting to it works and after that it can also be deleted, all while the VM is paused.) Thanks!