Not immediately apparent to people new to the site.
Slow to load on mobile devices according to everyone I've talked to and in my personal experience.
Not always responsive.
Hard to toggle on a small touch screen.
I don't think anyone would have a problem with two search bars next to each other or with one right above the other. No toggling, sliding, or selecting... just two distinctive search options.
If you insist on only having one, please make it easier to select them and more visually intuitive.