The 1:14 twist on the first models to hit the battlefield were a major issue as well as no cleaning kits were issued as the military thought it would never need to be cleaned. Later on when the inaccuracy of the M-16 became a problem because of bullet instability that caused it to tumble, the twist rate was changed to 1:12.Yeah...great idea. Take a round that won't chamber for whatever reason and force it into the chamber. What could possibly go wrong?
The gun was just fine in the early years. The ammo was the root cause of the problems. Once that was sorted out the gun's been just fine. But it still needs to be maintained like any other piece of equipment.
As you said, the wrong ammo creating gunk in the receiver caused a lot of American lives. Soldiers were found dead with a bolt in their lap and a rag in their hand trying to get their weapon back into the fight with a field cleaning.
The forward assist can also be used to silently chamber a round when slamming the bolt home may give away your position. It was a great addition.