Pro's you can offer your customer inline effects that others can not.
Cons. you lose 2 print decks when running the foil so if you want to print four colour process with couple of spots you need an 8 colour press.
Some foilers have to use a complete repeat of foil even if you are only foiling a small area, pick one that can run narrower width foil rolls.
I have never worked on one but have seen a few running.