I have a few piston setups and DI .
Here is the thing regarding cleaning, you can't argue the fact that piston systems leave the carrier / bolt / upper receiver much cleaner .
Having said that so what if its dirty ....unless you are OCD I seldom clean my DI .
I have KAC SR-15 that I have documented 800 rds and I have made a commitment to refrain from cleaning and guess what , it keeps running and running. When it gets sluggish to a point where it won't lock back on empty mag for example, I pull out the BCG and lube it up or squirt CLP into vent holes. i want to see how much crap I can get stuck on the bolt over time haha
I have a LMT MRP Piston 10.5 upper and it cycles anything with over 1000rds no cleaning . I do pull out the piston assembly to look at it but haven't cleaned it yet. It's nice that the BCG is much cleaner than my KAC but so what . My factory MRP 10.5 upper would not lock back on emply mag with anything less than full power 5.56 pressure stuff. The piston MRP runs everything fine so seems to be a plus for the piston version. Please note that the DI 10.5" LMT uppers have relatively small gas port ( hence not full cycling with 223 ) since it was deigned to also run with a suppressor.
I have 7.62X39 upper with a Adam Arms carbine length kit . Took some tweaking since the AA kit is not suppose to work with the 7.62X39 out of the box . Once it was running though it keep chugging away with everything form handloads to factory US made, Com block stuff, to underpowered Wolf crap . When it was a DI it always had cycling issues with the Wolf . Most of this surplus burns dirty but BCG squeaky clean . I am very happy I did piston conversion for the 7.62X39
I also have a toy Adam Arms factory built 7.5" upper that just runs and runs on anything I feed it. Never been cleaned over 500+ rounds . I have put together two other 7.5" DI , and one ran everything fine, other would short cycle with low power stuff. I have never removed the piston assembly for cleaning either .
IMO if you are contemplating setup for a Grendel bench gun I think it wouldn't bring much benefit. If its like my 6.5 Grendel my round counts are much lower since I don't run and gun with it since very Lapua brass I loose brings a tear to my eye . Also you would most likely be hand loading , may of the go to powders such as CFE223 , VV N530 etc all burn clean .
I barely have time to go shooting , and load ammo so really don't want to spend it cleaning so I like my piston uppers for that reason. Certainly not a necessity as my DI SR15 has been going strong with no cleaning, its just a bit messy inside but so what.