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I find mega drop-downs actually increase page views.
because Google was seeing the anchor text in the drop-down items as being potentially relevant to the page itself. But the visitor would land on the page and at least some of the words in their search were not visible unless they hovered on the menu
But those menus can often make any search engine's job harder. And that means some content may not rank as well as it could, and other content may rank well for poorly targeted queries.
just trying to understand where the difficulty would lie
I guess the large Brand Names would not be affected by a Mega Menu