I had an incredibly hard time aligning my rear MT-800 rotor using GRX400 hydraulic brake calipers. Enough so, that I ended up going back to cheaper/heavier non-ice Shimano SM-RT10 rotors. The front GRX caliper aligned fine with MT-800 rotors, but on my rear I tried every technique and was still getting rotor rub. I even swapped out the brake pads, reset the calipers multiple times and tried centering the caliper. Also the rotors appeared to be fairly true, but the MT-800 appears a tad thicker, enough to cause rubbing. Once I put the SM-RT10 rotor back on, there was no issues whatsoever. It sucks, because I'm using the MT-800 rotors on my road bike which are using TRP HY/RD cable hydraulic calipers and the braking is better. There's less squeaking, there's a nice weight savings (180g vs 108g) and visually they look nice. The difference between the calipers though is that I can adjust the TRP pad spacing, while I can't adjust spacing on my GRX400 calipers.

My opinion is that if you're using them on mechanical or cable actuated hydraulic brake calipers, they're highly recommended. If you're planning on using full hydraulic brakes, I'd definitely consider other style rotors.