I think they're all hard. A lot of horses simply are not at their very best at that distance. It's too far for them. Not only that, but the Belmont is at the end of a very grueling five weeks, and some horses endure better than others. That's basically it.
Some horses are just able to handle pressure, mentally and physically, better than others.
And it looks like Charismatic is doing it. He appears sound and he looks terrific. I saw him Wednesday, I was with him for about an hour. He looks like copper. He hasn't lost much flesh since the Derby and Preakness.