Are GNOME Shell extensions compatible with Unity?

No. They are only for gnome-shell. They are expecting a special API, are written in javascript, and in general are too different to be used on Unity.

Note that the idea of writing most of the gnome-shell UI in javascript will make it very easy to write extensions that can hook near everywhere. Of course it will take a penalty in speed. It's quite similar to the trade-offs between Firefox and Chrome. Firefox UI is written in XUL (an extension of javascript) and Chrome in C++. The former is way easier to extend, but the later is likely going to be always faster and more responsive (I'm talking about the UI, not the rendering or javascript engine).

So to answer you second question, no, likely Gnome-Shell is going to be always more customizable than Unity as a result of a design decision.