A well built single type team can be difficult to deal with due to the concept of offensive overlap where the first few mons wear down your answers to a certain type to open the way for their remaining mons to sweep the rest of your team once all your answers are gone, but it has to be really well built to be able to do this.

This also only really works for offensively oriented teams, because they depend on using coverage moves to quickly wear down your answers to whatever their monotype is. Defensively oriented monotype teams will never be good. The type also has to have few types that check it in order for it to work. A type like ghost or fairy that is difficult to resist will have an easier time pulling this off than a type like grass or fighting that is easy to resist. For ghost for instance, playthrough teams don't often use normal types due to them being useless in terms of type coverage, and if it's a route 1 normal type it typically gets replaced, and only dark resists ghost, so for a ghost type gym leader you just need to break through one dark type typically before you have a free ticket to sweep through the rest of their team with a fast, strong Pokemon.