You can always revert to 11.x, provided you do not upgrade the pool to 12.x
When I transitioned to 12, I purposely nuked my pool and reconstituted it with an SVDEV, so there is no going back for me. FWIW, I had no issues other than some odd ACL problems, mostly due to me. Thanks to
@anodos, those were resolved quickly. However, I have a very simple setup here, SMB only, no plex, VMs, jails, or other add-ons running.
The reason I rebuilt the pool from the ground up is to fully take advantage of SVDEVs. Otherwise, I'd have to move all the data around the pool at least once to transition metadata storage from the HDD VDEV to the SSDs in the SVDEV.