I have not fired an AR15/M16 since the miltary many, many years ago. Old A1 version. I loved the carryability, light weight, and accuracy. However our old ones jammed constantly. If the reliabilty has been improved, which I hear has happened, it would be a fine combat rifle. BTW they are very, very easy to take down for cleaning.
If you're hella old and are talking about the the original Army-issue XM15E1s or Air Force-issue M16s, then yes... they had some serious problems. Most were due to bad powder in the brand-new 5.56 load, but lots more were due to an unrefined design. When the first order was made, they were ordered as-is, civilian sport rifles, mostly untested, unmodified for military use. After putting them to use, they realized the ammo was no bueno, first of all, and that some modifications were going to be needed to produce a legitimate fighting weapon. Once those modifications were made- hotter ammo, chrome-lined chambers/bores, buffer, forward assist, faster twist rate, etc.... Thus, the M16A1 was born, and it's been a very reliable, very accurate, very lethal platform ever since. They've been peripherally reconfigured a few times since, but the major mechanics remain almost identical to this day.
Liberal application of lube and lots of ammo.... all they need to stay happy.