I sometimes think it's not that there are more difficult than others. It's more that we are so familiar with their faces because we've seen their movies so many times. Doubly so with Han / Indy.
I think Robert Downey Jr. is at that level of familiarity now too.
Maybe. I dunno.
That might be part of it, but they also have a couple of faces that seem to look very different depending on the expression or angle or lighting. And they don't really have the typical square-jawed Hollywood look either.