Would be creating menus totally in views reasonable? Above question is about two situations:


Standard, simple menu with 1-2 levels of depth,
Catalog on commerce site, with number of products, next to every item (taxonomy term) of menu.


I can imagine, that counting all nodes related to every taxonomy term every time when site is accessed would kill the server (if there is like hundreds of products), but views have some cache option, don't they? Can they cache their output and make menus views driven reasonable?
Yes. That's the point of views, you should not worry about performance at that point, just build the menu and if later you find that performance is worse than expected start taking a look at this question, there is a big amount of information on views caching. If you need more you can also go here to find more information on caching and performance in general.