One of the problems is that you might not use the target language at the equivalent level of abstraction. For example, C is a popular target language, but the C emitted may be very unidiomatic and nothing like human consumable code, it's not all that unusual that a language compiles all code to one big C function where the function calls in the language are jumps, which is a way to get around the limitations of the C calling conventions and stack.

The same thing applies to compilation to Javascript, the resulting code may use a tiny subset of the language.

I don't like the word transpiler, because there is nothing useful about the distinction (unless you count people using it to denigrate compilers that doesn't target traditional machine code).

I could see the case of using it as a name when the transformation is reversible, like you could probably turn Javascript back into Coffeescript.

What value does the word have? When I'm writing a compiler, it doesn't matter whether I target C or asm, or Javascript, as my output language. I'll still write it the same way.