I simply disagree that difficulty has anything to do with a player getting to 99. It is a grind, but your character is able to do pretty much the exact same thing at 90 for most specs.
Now, if we talk about being the first player of a given class, it actually becomes a test of difficulty, because there are speeds at which you need to clear certain content before your char is virtually indestructible, and hitting fast chaos can be tough with certain classes.
But whatever, it’s a minor semantical nitpick. This game isn’t hard unless you make it hard on yourself, by imposing goals or time restrictions. First hardcore char to “X” for example, can be more difficult, but 99 on its own is not a difficult feat, just time consuming.