There are all sorts of ways of looking at ergonomics, the efficiency work or design. What is 'efficient' can vary on perspective, but it all will come down to sustainability. How well is efficiency maintained for the period of time during which the work is done or device is utlized?
The M16 does well with the hand controls, no doubt about it. As with any design, it can be improved, but does well as put into service. However, it has one flaw disliked by many and it was done to be more ergonomic, the high sight system. Clint Smith would argue against the efficiency of the M16 due to this and that is because of getting "canoes" put through your head when the enemy shoots you there because your head sticks up higher with the M16 than with many other guns. Being dead destroys the sustainability of action in real life application (battle).
As with the 1911, there are folks for whom the grip and grip area controls are not easily manipulated. Modification of the gun makes that more possible.
Beyond the high head target complaint of Smith, most any action on the M16 can be performed repeatedly and comfortably over a long period of time and so that would qualify the rifle as being nicely ergonomic for most folks. Something to not here is that as with shoe sizes, there is no universally accepted single size that fits all. Just because some people find the gun ergonomic does not mean it will be to everyone.
"If you look through your scope and see your shoe, aim higher."
-- said to me by my 11 year old daughter before going out for hogs 8/13/2011