Even though I really like postfix from an elegance standpoint, and I use an RPN calculator, IMO it's harder to reason about subexpressions with postfix. Being able to decompose an expression into independent parts is what allows us to understand it. If you just randomly scan a complex expression in infix, if you see parenthesis or a +, you know that what's outside of the parenthesis or on the other side of a + can't affect the part you're looking at.

If you're executing the operations interactively, you're seeing what's happening on the stack, and so it's easy to keep track of where you are, but if you're reading postfix expressions, it's significantly harder.