Seems like the general consensus is that the direct blowback from a 9mm BCG is more violent and will in a sense wear down a standard AR15 trigger.  Seems like certain robust AR15 triggers will work on the AR9 platform depending on if the 9mm BCG is ramped or not.  Besides functionality, and more of a durability/over durability standpoint.  I am worried if excessive usage of a 9mm/PCC trigger will cause issues if used in a .223/.556 platform.

I have a lower Aero Precision M4e1 I plan to use AR9 most of the time, with the occasional switch with an AR15 upper to use .223/.556  I am going with the CMMG RDS 9mm BCG and barrel setup, since the BCG is the same length of a standard AR15 bcg, CMMG states that a standard carbine length buffer tube and carbine weight buffer be used with their RDS system.  Furthermore, I also plan to use 9mm endomag converted PMAGS, which are basically Magpul PMAGS with the internals converted to use 9mm.

So with the above setup, essentially this would allow me "universal" use with an AR15 upper since there is no need to switch out buffer for a heavier 9mm buffer, and vice versa.  Magazines will be the same.  So from a lower standpoint, the only difference will be the 9mm/PCC specific trigger.

Perhaps with a pure blow back system, especially if not enough bolt weight, buffer weight, etc which will put extra stress on everything.

I might worry about it some on a registered FA receiver or some other super expensive/impossible to replace receiver, but only worry enough to ensure it is set up properly.

With the CMMG set up, I'd worry even less.  

One thing to be aware of though - the "regular" EndoMag inserts are different from the CMMG-RBD EndoMag inserts (the "standard" ones have the ejector on them, the RBD ones do not)