I read Graham's stuff interesting. And from what I understand the slant head is fine. The squish ring or whatever it is called is still in the center of the piston. So detonation and what what not still happens in the center. just because the spark plug is angled does not mean that the explosion is now going to be slanted, it should still be contained in the squish ring, so uneven heating and piston wear would be a result of some other factor. If you reduce the height of the head gasket, and or use a longer spark plug in an effort to increase compression, this could cause problems because it could possible remove the ignition source from the center of squish ring enough to cause some sort of uneveness in detonation.
What I just said could be total crap, but it is my best understanding based on what I read and what makes sense to me. Please feel free to correct me, as I am in no way an expert.