[kccqzy]

Why would you need to enter the deepest expression first?

[7thaccount]

I think they mean that for a complicated expression you have to spend more brain power figuring out how to input things with RPN. It is a little more common sense with infix.

[billmcneale]

Yes, exactly.

If you have

1 + (2 * 3 + (4 / 5))

The smallest key count with RPN is to start entering the deepest expression first:

4 ^ 5 / 2 ^ 3 * + 1 +

You can type it left to right but it takes more keys and is probably on par with standard paren arithmetic, which negates the RPN advantage

1 ^ 2 ^ 3 * 4 ^ 5 / + +