Subject came up elsewhere in the context of why the AR has a NRCH vs the other choices Stoner could have made. (I'm specifically not talking about kludges that add a RCH to the AR, those are just silly.)

My take is that from a design standpoint there are several compromises to both:

RCH advantages:
1)Simpler to implement. Can be made unitary with the bolt carrier (AK) or a simple removable piece (Sig 550, FNC).
2)Typically stronger because they are bigger and more robustly engage the bolt carrier, if it's a separate piece of steel at all.
3)Eliminates need for separate forward assist.
4)Useful for diagnosing what's going on inside the gun. If you have stoppage where that RCH has stopped will tell you a lot about what caused it.
RCH disadvantages:
1)Snag prone. If it's big enough for the trooper to get his hands on then it's likely to catch on things.
2)Possibility of causing stoppages. Get that RCH too near cover (or your thumb) and you will make the rifle quit working.
3)Requires a slot in the action. The slot either has to extend all the way to the rear take down point (AK, FNC), be in a open action (M1 Garand and carbine, Mini-14) or the CH has to be removable (Sig 550, SCAR). The slot is a way for dirt to get into the gun. The slot can be closed by a spring loaded plate (FNC), rubber lips (Sig 550), or by the bolt carrier (SCAR).

NRCH advantages:
1)Can be made folding easily, reducing snag potential.
2)Nothing is moving outside the gun, so hitting the CH during firing isn't a problem. Operator can't get caught up in the moving CH.
3)CH can cover the required slot (Uzi, FAL).
4)CH is always in the same place for user to grab.
NRCH disadvantages:
1)More complex than RCH. Even more parts if CH incorporates FA function. (AR CH part count should really include the FA part count if you're comparing, since the AR splits those two functions).
2)Typically not as strong as a RCH. NRCHs just aren't as robustly made as the parts are smaller. You might be able to kick a FAL open a few times but don't try that on a AR.
3)Not useful for diagnostics. CH is always in the same place so you don't have a additional clue as to what type of stoppage you have.
4)Requires additional manipulation to engage FA function. Push, turn, pull, there is almost always additional activity required to get a NRCH to engage as a FA.

Personally, I own rifles with both RCH and NRCH. Both designs are a compromise. I see the advantages of both, but both have drawbacks too.

H