[samatman]

The advantage is that the language isn't stackful.

This is about more than just underflow, there isn't a stack at all. Missed arity is a syntax error, not a mistake in reasoning about the program state.

I'm not sold on Om's solution, I doubt the author was either since it's frozen in time. I have long wondered about a good prefixed concatenative language, without really getting anywhere. Om seems too much like inside-out Joy, although I'm not sure I could justify that impression in detail.

[mostlylurks]

Stackfulness and the possibility of stack underflows are not actually constrained in any significant manner by whether or not a language uses prefix or postfix notation, and it would be more-or-less trivial to reject arity mismatches in a postfix language as well.

The one real advantage that prefix notation has is that it is familiar, since it is so widespread in mathematical notation. Though that particular advantage would optimize the experience of using a language for people new to programming or that particular language, whereas it might realistically make more sense to optimize for the experience of people already familiar with a language, since those will constitute the vast majority of development time in any serious language, and I would argue that postfix notation has an advantage there in most domains.