I was a Small Arms Repairman for the Army 69-72. Worked in a depot shop with 100 guys. I ran the testfire range. The early M16 problems were due to no cleaning equipment issues and ammo with too much preservative in it. Crudded up the rifle quickly. These problems were quickly solved. The original guns had standard steel barrels and the throat wore out quickly on full auto. The worn barrels tipped bullets badly and I think the legend of the "tumbling bullets" came from that. Clear sideways hits could be seen at 25 yard paper target. To correct this they added a stellite liner to the chamber that extended up into the barrel a ways. Around 1970 they came out with the chrome lined barrel. We always replaced the plain steel barrel when we found one. Magazines must be checked and replaced often. They are a bit fragile. Properly cleaned and lubed M16s worked well in the jungle. I dont know about dusty/sandy conditions.
The military went with the M16 for a number of reasons. One was that the troops did poorly with the M14 on the range. Out of some 220 guys in my basic trainning unit, fully half of them failed to get the 30 hits out of 84 targets required to qualify. They were afraid of the recoil. These same guys did a whole lot better with the M16. Another was weight. You could carry a whole lot more ammo with a lighter rifle and ammo. Cost was another factor. The Gov was paying 117.00 for an M16 and the M14 was 177.00. Ammo for the M16 was .07 a round and M14 ammo .17 a round.