Are you comparing mergerfs to BTRFS RAID?
Do you know what the status of BTRFS RAID is? In the versions of BTRFS RAID where it's possible to remove a drive AND consolidate data reducing the array in size (RAID 5 and 6), the current status is "unstable". On the other hand, what you're describing is not possible in BTRFS RAID5. Removing 1 drive of 3, would result in fewer drives than is possible in a RAID 5 array. That would require a minimum of 4 block devices.
((The command in question is btrfs device delete . In theory, the command triggers a data rebalancing before the drive is removed from the array. What I find interesting is that the command doesn't work for bad drives because the ability to "read" the drive is a requirement.))
With that noted, did you know that you can set up BTRFS RAID in OMV, in any implementation available in ROCKSTOR? What you may not be aware of are the files systems and RAID flavors that OMV has, that are NOT available in ROCKSTOR. Further, OMV has more value added plugins than ROCKSTOR and and an easy to use, thoroughly tested, Docker GUI.
Of the serious "out of the box" candidates for a Home or a Small Business NAS, during the last 4 years or so, I've tested them all. If you look closely at Rockstor's "features", currently, there's more in "Beta" than in "production". These features have been stable and available in OMV for years and I'm using several of them of them right now.
For these reasons and many more, ROCKSTOR can't even be compared to OMV.