I have a Barrett REC7. The total weight of the entire piston system is 3.3 oz. It is much, much cleaner operating than the DI gun I have.
You can argue theoretical reliabiity advantages as long as you want - there are pros and cons for both systems. I bought the piston gun because I wanted to see for myself if there was an advantage and not just read the opinions on Internet gun forums. Other than the super clean operation of the rifle - that's about it so far after having it for a year.
I've cleaned the gun once in 3K rounds and only because I wanted to disassemble it at that point to get an idea of what was going on with it. The piston in the REC7 is self cleaning - so even that doesn't get fouled and need cleaning. I dipped it in Carbon Killer and wiped it off - and a little fouling came off of it - but really not much. The white rag was just slighty grey.
The bolt / bolt carrier are unique to Barrett, but so is the bolt / carrier on a KAC AR. I understand the Erector Set interchangeable parts commonality fascination with AR owners - it doesn't matter to me - so I don't find that a liability with the REC7.
As to all of the "what if" parts failure scenarios - if I was that worried about it, I'd buy a second bolt, carrier, and piston system.