The AR-15 design commits the sin of compromise and pays for that sin dearly, when it is used as anything other then a full sized assault rifle. The AR-15 turns into a mechanical failure of mediocrity when tailored to other roles that are better suited to specifically designed firearms.
Biggest problem IMHO with the AR-15 design and relatives is that it uses direct gas impingement onto the bolt/bolt carrier. That means the working parts get insanely hot, very fast, and get badly fouled with sustained fire. The AR-18 design and relatives uses a gas piston arrangement so avoids this problem, and generally stays a hell of a lot cleaner.

When you need a compact rifle that is AR-15 derived the barrel length is cut to carbine length or worst of all PDW size, which obviously decreases accuracy. When the AR-15 is sniperized using 5.56mm, the round itself becomes a limitation when the use is switched from DMR mode of fire to Sniper.
Accuracy isn't really an issue with a PDW - frankly anyone who is issued one isn't going to be able to shoot well enough to make full use of the capabilities of the weapon anyway. Far more serious is muzzle velocity - the 5.56mm NATO round relies pretty much entirely on high impact velocity to cause the round to fragment on impact at the cannelure. With a full length M-16, SA-80, G36 and the like this will reliably happen out to over 300m in most circumstances. With the M-4 carbine IIRC the distance is getting down below 100m (note the number of complaints recently about the hitting power of the M-16 family - I haven't heard a single one from a UK based soldier, who will have been using a full length weapon), and with PDW derivatives it may well not happen at all. This drastically reduces the wounding potential of the round, which is MUCH more critical than any accuracy limits.
All that is necessary for the triumph of New Labour is for good men to say nothing whilst CGS.