Here's some more information resulting from my attempt to boot OpenWRT on my RT-AC87U.
Silly me didn't backup all the mtd partitions before doing this, lesson learned.
It appears that the install destroys the jffs2 (partition named brcmnand on the 87U) file system and it isn't possible to recover it using asuswrt-merlin (or stock) firmware.
So any functionality that depends on it no longer works, such as the Traffic Analysis system and others.
I suspect the partition named asus (I don't know what this contains, perhaps device specific asus info) has gone the same way.
I also suspect, but aren't certain, that this causes the asus firmware restoration utility to choke and refuse to restore firmware, I'll need to do more on this some time later.