The valve springs. The pockets that are cast in the heads can only be cut so far down and do not allow a spring to be installed at the proper height. AFAIK, springs that (generally) allow more than say .525" lift are supposed to be installed at 1.8" or so. The RHS heads seem to only allow springs with a max of around 1.725" installed height. Therefore you can't utilize the flow potential these heads offer since you can't use a cam that will take advantage of the potential.
Obviously there are work-arounds to the issue, but that's why I say if it's a max-effort build, why start with these? Nothing is perfect OOTB, but this to me is not a good thing. Money would be better spent elsewhere rather than trying to fix a poorly designed product that wasn't designed for max effort anyway.