The left shift on an ISO keyboard is the same distance from your left pinky as the right shift is from your right pinky. I don't see how it would be (objectively) harder. Plus with an ISO keyboard you get to move the `~ key down closer. I prefer to program on a US-International / Dutch Apple keyboard. It may be Stockholm syndrome from having to work with German keyboards.

If Wikipedia is to be believed both variants were introduced at the same time by the same company (Key Tronic). The European one fits one key more.

I never had problems to hit the European-style return-key for what it's worth and need a long time to adjust to the US variant whenever I have use them.