From what I've read (assuming I understand correctly) they are a low-ish
vibration drive with some fancy head positioning for alignment... but
should be limited to 5 at most, or at least are intended for upto 5
drive systems, so I wonder if this means that more than 5 could suffer
from vibrations throwing disks out?

No.

RED's are a consumer drive with TLER enabled so it plays well in small
NAS units. To protect their enterprise sales, they put an artificial
limitation of 4-5 drives per NAS in the warranty terms.


-- 
Drew