This seems to be not an issue with Fable overload suffixes as I thought at first, units of measure are correctly being represented in the hash. However because you're using the inline trick to resolve the overload, Fable gets confused. F# witnesses should help with this. They seem to have been fixed in FCS so we can try this out after merging the feature.

You're probably aware of this but if we use "normal" methods, F# compilation fails because the signatures are not different after erasing units of measure.

type ToLength = ToLength with
    static member Make(x: int<px>) = String.Join("", string x, "px")
    static member Make(x: int<em>) = String.Join("", string x, "em")

// Duplicate method. The method 'Make' has the same name and signature as another method in type 'ToLength' once tuples, functions, units of measure and/or provided types are erased. (code 438)

A trick to work around this without inlining is to add a "phantom" optional type, which works with Fable. You can try this or we can wait for F# witnesses and see.

type ToLength = ToLength with
    static member Make(x: int<px>, ?_phantom: int) = String.Join("", string x, "px")
    static member Make(x: int<em>, ?_phantom: int * int) = String.Join("", string x, "em")

Oh that's an interesting idea. I'll try it out when I get a chance.