Somewhat? It's a bit of both imo. For a boss to be really difficult these days, it needs to be complex. Players have become better over time, and that's fact. But the first time I thought Blizzard had officially shit themself was on Klaxxi. 9 bosses in which they usually have 3 - 4 abilities each. The tactic based on all the different mechanics was so difficult to explain, I basically just tried to explain on the go as we progressed on it. Bosses have only continued to become more complex, although less abilities than Klaxxi.
The only bosses that resemble something you'd see from Vanilla in WoD would probably be Fel Lord Zakuun and Butcher mythic. They are fairly mechanical, but the sole difficulty lies in the numerical difficulty. It's not hard at all to grasp either boss. It's almost a basic tank and spank with a few shit mixed in, but the few shit mixed in can usually screw a kill. Fel Lord is/was usually nerve wrecking to do on farm for that reason imo. It's just so easy to wipe on a numerically difficult boss.
Personally I prefer a mix of both.