You can't really know how difficult or easy something is for someone else to do, and it's a bit presumptuous and foolish to assume that you do.

You might have seen them do it.

how would that help?

In some cases, ascertaining the difficulty of a task for someone else is indeed ill advised. But for most cases, where you know the person in question and have a good idea of their abilities relating to the task in question, it certainly is possible, and indeed successfully leading people requires such an assessment.

I would say that successfully leading people requires knowing what they are capable of. However, knowing how difficult it is for them is irrelevant. Knowing what they will or won't do and how long it'll take them and how it will affect morale is different.

One can judge based on self-reports from the person in question if nothing else.

Yes, but it's like the pain scale. If I say my pain is a 4 on a scale of 1 to 10 and someone else says there is a 4 on a scale of 1 to 10, then you have no idea who is in more pain. I may well be in more pain than someone else reporting a 6, or less pain than someone reporting an 8.

It's especially bad when they tell you 1 is no pain and 10 is the most pain you personally have ever been in. I tend not to use that scale because it's so stupid.

"In general" strikes again!!