I can see the brass inserts being a potential problem, simply because the threads are softer than the steel screw so if you tighten really hard, you could strip the threads.
Some people see threaded inserts as a means of putting MASSIVE PRESSURE on the neck screws on the basis that this improves the tone, but I personally think that's a load of hogwash - IMO the screws only need to be tight enough to hold the neck firmly without shifting.
What's more of an issue is fitting the inserts in the first place - apparently if you try to fit them in hard wood without cutting a thread first (as shown on the Manchester Guitar Tech installation instructions), the outer threads on the brass inserts aren't strong enough to cut the wood and the brass can "fold" on itself.
When I used the steel inserts, I just drilled the pilot holes and let the inserts cut their own thread (using a bit of dry soap on the threads to stop it sticking). I was expecting it to be tough, but they went in easily - drilling the holes in the right place was the hardest part of the job.
It would be extremely difficult to get the inserts back out though!