If they are questioning the strength of the 28 tpi threads, there is no merit to it, I use 28 tpi threads on all of my Painkiller muzzle brakes
and if fitted properly, they will handle anything you can throw at them and thats including the 408 CT class chamberings.
If they are talking about the thickness of the barrel wall then I could see more reason to worry but even then, I have put several 1/2"x28 tpi threaded brakes on 7mm rifles with no problems at all, infact I have one on my 7mm Allen Magnum right now and it has about 700 rounds down it and no problem at all.
It all comes down to a very quality fit and I do not put a thread relief on the barrel after the threads which further weakens the barrel threads, use a thread relief on the brake threads if you need one.
Obviously, you want to use the larger diameter threads that your muzzle diameter will allow, especially with a button pulled barrel as the bore diameter will increase in size the more you turn the OD down.
I have never seen a problem with 7mm with 1/2" threads. 30 cal would start to worry me.