Are they really harder than landing on Mars?
Sorry, but with all respect, that’s nuts. We know all we need to know about asteroids to plan a mission to them, including the space radiation environment. All you need to do is pick one. And landing on Mars is hard.
That’s not to say that I think that NASA should be doing an asteroid mission, of course. And especially it shouldn’t as long as it continues to spread the nonsense that it needs the SLS/Orion for it.
[Update a while later]
I think that this is mostly FUD from people who don’t want to do an asteroid mission.