The 6-holers seem to me to be pretty sturdy, especially compared to the 2 cavity models. I have not used these extensively yet and the only potential problem I see is that the bolt that the sprue cutter pivots on has a tendency to loosen. I may have to drill and pin that bolt, or simply disrupt the threads slightly. I'm thinking on the aluminum, it would be better to pin the bolt.
I used some of the Lyman 4-cavity molds back years ago, before the widespread availability of commercial cast bullets. They made perfectly good bullets, but boy, were they heavy after a while. The aluminum Lee molds don't fatigue me like the steel molds do. I've seen some 8-cavity H&G molds. I can just imagine what a casting session would do to my arm muscles with that one!