"So why does this work if you use a sweep along a guide creating exactly the same geometry?"
Because it isn't creating exactly the same geometry. It is only very similar, but not the same.
It probably works better with a sweep the same way general patterns fail less often than identical ones for geometry reasons. A sweep is probably slower with better error handling.
If PTC can't handle Generic -> Family Table Entry -> Material -> Appearance, which is simple book-keeping, PTC won't be changing geometry generation and the underlying difficulty of solution accuracy any time soon.