My experience with PMAG's, Gen 1 and 3-
BCM upper on a Spikes lower. No issues.
Bushmaster upper receiver with quality parts attached on a Spikes lower- No issues
BCM upper receiver with DD barrel and BCG on a Spikes lower- No issues
In the future, don't take no for an answer. Ask to speak to his boss and go higher until you get the answer that you want. In this case the customer should be right.