Relatively high energy cost since you’re undoing an endothermic reaction, you need to do a lot of it since we use water in large quantities… but most of all, the planet naturally does a lot of desalination for us already through various geological processes, so our “price point” for desalination is $0 per liter (infrastructure to capture rain, dam rivers, or tap groundwater isn’t literally free, but it’s pretty close - especially when it comes to the marginal cost for the next liter). It’s not difficult to desalinate per se, it’s difficult to desalinate extremely cheaply and at huge scale.