There is no UEKF. There is UKF. Yes, it performs better than EKF/MEKF. And yes it's computationally expensive for that matter. And yes, people are looking into SPUKF/ESPUKF to reduce that computational overhead.
But again, follow the golden rules: "If it ain't broke, don't fix it". Typically these filters alone are an illogical point of metrics from a mission perspective. This whole sensor fusion thingy is coupled with hardware. Good hardware + shizzle software = functions, then viola! Shizzle hardware + awesome software = still doesn't work, then "insert hello darkness my old friend here". Random puns apart, there have been precision pointing mission in the past that got away with just a KF and PD, only the control bandwidth had to be pumped up to 4Hz from 2Hz.
It's very mission/hardware/satellite specific. Check out the classic paper: Survey of Nonlinear Attitude Estimation Methods from hotshots featuring: John L. Crassidis, F. Landis Markley and Yang Cheng. Blue Canyon Technology from what I understand runs a 6 state MEKF. If it works for them, it could easily work for a rigid body mission for you.
P.S. Sorry for the informal lingo, had too much caffeine today. Have a good day :)
Edit (in progress):
I realized there is indeed a UEKF. Never saw it applied on a sat mission before. Shall dig it, in free time, and come up with a rather constructive answer. This Paper claims that UEKF performs better than UKF, and has less computational time. Interesting. This is not a paper on ADCS, but target tracking instead. Different frameworks.