The results vary between annoying (need to restore / “resilver” and have no redundancy until it’s done; massively increased risk of data loss while doing so due to heavy IO load without redundancy and pointless loss of the redundancy that already exists) to catastrophic (outright corruption). The corollary is that RAID invariably works poorly with disks connected over using an interface that enumerates slowly or unreliably.
Yet most competent active-active database systems have no problems with this scenario!
I would love to see a RAID system that thinks of disks as nodes, properly elects leaders, and can efficiently fast-forward a disk that’s behind. A pile of USB-connected drives would work perfectly, would come up when a quorum was reached, and would behave correctly when only a varying subset of disks is available. Bonus points for also being able to run an array that spans multiple computers efficiently, but that would just be icing on the cake.