The big issue is that an AR15 is designed around a 20" barrel with a rifle length gas system and a certain gas port size. When you start changing those parameters, you change the way the system was intended to work and weird things can happen. brassshower described the issues well.
In SBRs, you have a shorter gas system, higher pressures at the port and chamber and a very short dwell time after the gas port. All of these things combined generally mean that an SBR has a smaller "window" where it will run reliably compared to the longer barrels.
Finally, you have to remember that as the barrel heats up, the time it takes for brass to shrink away from the chamber walls increases by microseconds. This mean resistance to extraction increases as the barrel gets hot on any AR - on an SBR, you are already at the edge of function with a normal extractor spring and you have less barrel mass, so you'll hit higher temperatures with less rounds fired.
SBRs can be made to run reliably; but that "window" where they run reliably will never be as big as it is for its longer barrelled cousins.
That said, my 12" SBR has been one of the most reliable AR's I have owned.
That's like cheating though because you have between 1.5" and 0.5" more dwell time than most SBR ARs and I'd bet the uppers are MSTN which is definitely stacking the deck reliability-wise
You can make a reliable SBR definitely; but a lot of people don't. Even LMT had lots of issues with their SBRs at first until they got the hang of it.