No personal experience, though I'm considering a BLR takedown in .243 and have, therefore, done some research.
What I've stumbled across is two things, which I believe to be true -
1. Both the steel and alloy receivers have bolt lock-up in the barrel, so there is no difference in strength. That is, the alloy receivers can handle the same calibers the steel receiver can.
2. The alloy receiver may cause some problems with scope mounting if care is not exercised, as the holes are drilled and tapped into alloy instead of steel.
So, it looks like there really isn't much to worry about with the alloy receiver vs. steel receiver.