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.
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.