mega prims are nasty, period.
They're lag monsters, they block pathfinding systems (not an issue with the kittycats I think), etc. etc.
They're as bad or worse than poorly made sculpties...
And so many sim surrounds are BOTH mega-prims AND sculpties!
Dunno about badly- vs well-made sculpties, though. Sculpties were a slimy hack. I figure they're all badly made because 'bad' is as good as it gets with sculpties. Unfortunately, we're stuck with them because there are problems you can solve with sculpties which are simply impossible with mesh (breedables being just one example).
What I love to hate is mesh which is nothing more than a quick conversion of an old sculptie. That's like, "see, it's 100% mesh, isn't that good?" when it's actually "yes, I know this sucks, but you want mesh, so here it is. sure, it still sucks, and is massively more expensive, causing more lag at all levels, and even introduces more problems for my users, but, hey, it's mesh, so shut up and shell out more money for this crap"