Most of the reason boarding ladders are physically harder to climb is the angle is wrong.
Boarding ladders are vertical at best, Usually they are at a negative angle. Also you often find they lack toe relief so it is harder to get your foot on the rung.

If they are rope it is even worse.
They will tend to pull out at the top while letting your feet go toward them and pushing with your legs just shoves your feet farther away from you while you end up hanging from your arms, almost like you were lying on your back and trying to pull yourself vertical.
This also shoves the ladder against the side of the boat at your foot level and makes it hard to get your feet into it.

Some people who work on rope ladders tend to put the rope between their legs so they can put their feet on the rungs from both sides, they find them easier to climb like that.

Another problem is that most boarding ladders do not have any projection above the gunwale. On land you usually set a ladder with at least 3 feet of ladder sticking up above what you are climbing.
This makes it much easier to step on and off the ladder.

Edit:
Philip got his answer posted up between the time I started to answer and got mine posted.
It is a good answer and obviously I agree with him.