I have fired many many many thousands of rounds through my DI guns, including case after case of Wolf and even worse stuff, like funky green guatamalan surplus in water stained cardboard boxes. I have had very few reliability problems with 5.56mm AR15s that were not related to magazines.

You don't generally speaking need to clean ar15. As long as they are lubricated they will usually run indefinitely. Of course if exposed to dirt, sand, other foreign particles any semi auto firearm risks having problems.

For nearly all purposes I think pistons add nothing and have serious downsides. Generally speaking the piston guns all have different designs and parts. if something breaks are you going to be able to find spare parts? is the company making your piston system going to be around in 5/10/20 years?

If you want one because you want one, I would guess the LMT system is probably pretty good, but I have never used it. I have been perfectly happy with the regular old LMT upper I have been using since 2005