I have a LOT of crossover between RM1/RM2 and RMSS (tiny bit of RMFRP) and I'd have liked to be able to use more RMU with them, but some of the changes have made that pretty difficult.
For example, I love that Spell Law filled in a lot of blanks, etc, but some of the changes to certain spells have made using those lists more trouble that they're worth (like Haste) and I haven't dug through it in enough detail to see what I may pull out of it.
Once more book are put out and I can get eyes on them I'm hoping elements of RMU prove useful. If not, no big deal, I've been writing my own stuff since the 90's anyhow.