Comment by crote
I think it's a huge caveat, because it makes upgrades a lot less efficient than you'd expect.
For example, home users generally don't want to buy all of their storage up front. They want to add additional disks as the array fills up. Being able to start with a 2-disk raidz1 and later upgrade that to a 3-disk and eventually 4-disk array is amazing. It's a lot less amazing if you end up with a 55% storage efficiency rather than 66% you'd ideally get from a 2-disk to 3-disk upgrade. That's 11% of your total disk capacity wasted, without any benefit whatsoever.
You have a couple options:
1. Delete the snapshots and rewrite the files in place like how people do when they want to rebalance a pool.
2. Use send/receive inside the pool.
Either one will make the data use the new layout. They both carry the caveat that reflinks will not survive the operation, such that if you used reflinks to deduplicate storage, you will find the deduplication effect is gone afterward.