The original M16 rifles of the early 1960's weren't nearly as accurate as the A1 or A2. At close range, though, the little rounds did tear things up. Much like the early reports of the Soviet 5.54x39 rounds. There were some documented close-range shootings that revealed the bullet taking a strange path. These were, unfortunately, usually from draftees attempting to leave the combat zone.
The rifles themselves were incredibly sensitive to powder fouling, the presence of corrosion in the unplated chambers, rough chambers straight from the factory, and the flimsy magazines issued at the time. The use of ball powder also resulted in a much higher than design firing rate, and buffer failures.
The flash-supressors, of the open-end type, were also difficult to manuver through the jungle without fouling on the undergrowth. We also found that the things would wick moisture up the barrel via capillary action during movement through wet foilage.
The rifles would also fail to drain quickly after immersion.
As to cleanliness. The initial instruction stressed how clean the weapons operated, and that the necessity for cleaning had been greatly reduced. They issued one cleaning rod for every three rifles!
I went through three M16 rifles in November of 1967. All but one failed due to bolt-over jams. The one that didn't was traded immediately prior to firing to an ARVN Ranger for his M2 Carbine, a bunch of 30 round mags, and most of a case of ammo.
In the end, I went back to my original M14 rifle.
