I don't think that Classic is harder per say, its complexity comes from different aspects. It required much more time to do anything. The pace is slower, the mechanics are more tedious (more interesting for me personally), and this is why I like the game. It's slow, it's tedious but not as previous installments in the genre such as EQ or AC.
Classic might be harder in a sense that there is no free hand outs of epic loot for everyone and that you have per-requisites for entering raids. Also, there is the aspect of resource management, threat management that required raid coordination on a different level than "all run left, all run into\out of the pool,...". Mechanically raids and dungeons are easier in Vanilla, in terms of numbers they're not as you can't just steamroll through in a brainless no CC fashion like today.
I mean, while World of Warcraft Vanilla and World of Warcraft BfA both bear the name WoW, these are different games. It's comparing apples and oranges at this point.