Agreed - we wrote an ML style language in my uni compilers course and it was taught by the primary maintainer of MLTon (Dr Fluet is a great guy). Since it’s full program optimizing, the compilation took a while. He told us to give SML NJ a try for faster compilation but slower execution. It was a headache and only marginally faster compilation for our use case.
That said, it’s the OG so I give it some slack. I did enjoy MLton though, but it’s easier to do when the instructor wrote it.
Standard ML was my go-to language for many years.
I always found SML/NJ complicated both to compile and use, compared to...
...well, pretty much every other compiler: Moscow ML, Poly/ML, MLton, MLKit.