I have run into problems. I had a Colt M4 flat top upper built up with an Armalite fiberglass handguard. I fitted this to a new post-ban MT lower.
The Colt front tabs that hold the pivot pin were a tad long (as compared to Bushmaster and perhaps other mfgs. As a result the upper would not pivot over the lower. The Hanguard nut (large nut device fixing the handguard onto the barrel nut)would hit the tabs (or the tabs would hit the handguard nut)
I understand this is a Colt issue on how they use tolerances. It's to Colt spec, not universal spec. You may want to check into this.