Not true.
I have used 10.3"bbl in the field and as a private citizen and have not had any issue due to barrel or gas return tube length. The crux is in the tuning, but once its figured out, its no less reliable than a longer length bbl. The Mk18/CQBR platform has shown its capability in numerous theatres of operation over the last twenty years.
As an aside, I have a DD Mk18 Mod1 Civvie street copy that I've got 1000rds down the pipe on with only the shakedown field strip and clean and lots of lube. I've had one stoppage due to missing a weapons check before hitting the range, I ran it dry and it still put out sixty or so rounds before it choked.
-S.