The principle argument about drop downs fall into two camps, the designersÂ who are often for them because they are a good way of using space and making the layout “tidy”; and the usability types who are against them because they can be confusing, dysfunctional when there are tiered levels and hard to use for mobility impared users.
Example 1 of a mega drop down navigation box
Example 2 of a mega drop down navigation box
Remember to code with screen readers and assistive technology in mind. There can be issues for people using mobile devices or people using screen magnifiers as the enlarged part of the menu may appear to be the only part of the menu. One way to help is to have strong visual signals for menu borders so people can follow them when they are enlarged.
And remember the standard rules of accessibility, don’t make clickable items too small, or too gimmicky.
See more from Jakob Nielsen to understand the finer details of mega drop down accessibility