Sometimes the absence of features is itself a feature. SML is a much smaller language than OCaml and F#. Offhand, I can't recall anything it has that OCaml doesn't, although it does some things differently (e.g. structurally typed immutable records).

I'm not sure there is much reason to use SML itself today, given that it has stagnated since the late 90s, but it is an excellent language for teaching programming because you don't have to sweep anything under the rug. But if you want to use it, then it does actually have well-maintained pretty good compilers. MLton probably generates faster code than OCaml and F# in many cases. The main problem with SML is that there are not a lot of SML programmers around, and hence very few libraries, particularly for handling modern protocols and formats.