My only knowledge regards their 351 Cleveland heads.
They are worse than garbage.
A friend who is one of the world's foremost authorities on building 351C engines (Google "Dan Jones 351C" and you'll be reading for hours) has done numerous tests with all the various 351C heads available from Edelbrock, CHI, Scott Cook, Ford Racing, Blue Thunder, and of course the factory 2V and 4V heads.
Not only were the Procomp heads the worst of the bunch that he has tested, they actually LOST something like 40 hp when tested back-to-back against standard Ford iron heads! Plus, they were so poorly made that even simple things like the holes for the head bolts weren't machined in the right place, so they took hundreds of dollars of work to even be able to bolt them to the block, easily eating up any price advantage they had over proper aftermarket heads.
I won't even get into my philosophical objection to a company that appears to have no R&D of its own, and instead lives by buying competing products and then (poorly) reverse-engineering them and branding them as their own.
Again, I have no knowledge of Procomp heads on any other engines besides the 351C. But I know enough to know that I wouldn't take a set of these if they were offered to me for free!