It is the least leaky abstraction I have ever encountered, which is amazing given the fact that it is also blazingly faster than every other framework from React to Vue. Elm is a way higher level abstraction then jsx or Vue yet you don't need to understand the lower level details to produce more efficient applications.
The most leaky abstraction I have ever encountered is probably SQL. Optimizing SQL involves understanding what it compiles into; Elm there is no such understanding required.