Hello,
OMV just support standard linux RAiD, aka MDADM.
All disks are running at the same time (energy cost), no capability to access existing data outside the pool, no data integrity control (checskum) to fight against data silent corruption, RAID 5 black hole issues...
MDADM works well but can't do nothing against RAID5 issues.
OMV is a ready to use NAS OS.
To use FLexRAID on it, it needs avoiding using some OMV features like Disks management (all disks are monted in /media, it's better for rights mamagement at mount with fuse to use /mnt) or add new compopnents (jre, fuse...).