McMillan needs to be bed, where Foundation does not. Foundation also has a a fully profiled action inlet, not a v-block, which distributes the load more evenly and keeps the rifle from having minor zero shifts. Not to mention, the material is not only stiffer than fiberglass (and stronger than aluminum) it is great at damping out vibration and giving the rifle a super solid feel under recoil.
Foundation also does not sponsor anyone, and yet many of the top shooters continue to switch to and use them.