Erik Darling points out that VSS backups aren’t instantaneous and can block queries:
Ah, backups. Why are they so tough to get right?
You start taking them, you find out you’re not taking enough of them, or that they’re not the right kind, or that you’re not using checksums or compression, or that you’re not storing them in the right place, or that the storage isn’t redundant.
It’s just like, why won’t someone make this easy?
Then you read about VSS Snaps, and they look so dead simple. You don’t need your DBA Ph.D to use them.
And look how fast they are! Oh how they blaze.
Read the whole thing.