I'm not sure that's really a caveat, it just means old data might be in an inoptimal layout. Even with that, you still get the full benefits of raidzN, where up to N disks can completely fail and the pool will remain functional.
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.
Old data still works fine, the same guarantees RAID-Z provides still hold. New data will be written with the new data layout.
Also, if you don't wait to upgrade until the disks are at 100% utilization (which you should never do! you're creating massive fragmentation upwards of ~85%) efficiency in the real world will be better.
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.