I'd have to say that Zeromus is harder than any super boss FF7 up. I'm a firm believer in strong final bosses. I think that a final boss should be very hard, and test everything you've learned in the entire game, as he is the final challenge before the credits.
Big Bang is so annoying, and even has a slight damage over time effect on it. I haven't really seen whether or not the DoT stays in effect after he casts his Weak spell. If so then Zeromus has got to be a boss almost as cheap as Xagor.
After Final Fantasy V, the difficulty of the games just started declining.
I think today's FFs have gotten softer. When Nemesis in FFX is easier than Zeromus from FFIV, you know something just isn't right. What was once normal in difficulty has become completely optional now. What was the most recent Final Fantasy dungeon as difficult as the Ice Cave from Final Fantasy I?