well, it is a weakness, but the biggest weakness (I think) is just the block structure.....thin webs, 2 bolt mains, not enough head bolts......just not really built for lots of HP
they can be made to make lots of HP (Race Jace is getting at least 700....girdled, etc....dunno if he is using a different crank, tho)
they are pretty reliable in the 500-600 range with a girdle and or caps......anything more is a bit of a crap shoot IMHO, particularly if you have to run lots of boost and risk detonation
the main differences in a Stage block are web thickness, more main bolts, better oiling, more head bolts....lots stronger and the way to go if you want to make big HP