I prefer the aesthetic and never had a particular problem with stringing up slotted heads, 12-strings and mandolins included.

As a once full-time instrument repairer I don't see that the slotted design confers any weakness as the headstock is thicker. Generally speaking you get a more favourable angle too from post to nut.

Slotted heads are a bit more time-consuming and expensive to make. For some guitars, e.g. nylon strung, you can't really have anything else because of the size of the string roller and I expect that design has carried over from classical guitar to steel-strung.

RS