The civilian version of the AR has been plagued with commercial no-spec components;
Colt large pin lowers
Semi-auto BCG's
Semi-auto pocket milled lowers
Aluminum gas blocks
MIM or other inferior alloy parts
Chinese fasteners (YFS gas key screws, chinese gas block screws)
Comercial buffer tubes
The introduction of commercial no-spec components has lead to a lot problems in terms of interchangeability was well as just plain inferior and cheaply made no-spec components.
While there are a few enhancements and improvements over genuine mil-spec it is now possible to have a nearly all mil-spec carbine with the exception of the FCG.
What you get with mil-spec is what Mike Mihalski at SOLGW refers to as "predictability of performance". Meaning wear items and service life are reliably predictable vs unknown.