Surely by using Elm you may improve the quality of your program, but you restrict your staffing choices down to the <1% of frontenders who either know it already, or are capable of quickly picking up a Haskell-esque language.

Either that or you commit your client to huge up-front training costs.

Or how about the scenario that goes like: "The contractor who wrote our web app in Elm left last month, but we need some bug fixes or complex feature adding. Can someone fix it? Anybody? No?"

How do you sell that to a client?