There are useful articles online that provide insight on the downside of using WordPress MultiSite.
Unfortunately, almost all assume a WPMU that have dozens sites and/or that have several site admins.
Based on that, they warn about the multiplicity of the tasks for the newtwork admin, the risk of having several site admins, WPMU's impact on memory and database, and the like.
Are the cautions against WPMU apply in case of a WPMU with just a few sites (2-5) and with only one admin?
Imagine you are setting up website that covers Business and Arts in English and French languages. Thus, "four sections"; so that English readers would not be bothered by French; and business readers by arts; as much as possible.
* You will certainly need categories named "NewYork", "London", "Paris" for each section - four times. But you can't do that on a single WP site, since category names & slugs have to be unique(be it a parent or sub-category).
* You would wish to have dynamic home/front pages for each section. Especially - on single posts -sidebars in a language similar to that of the post being read.(Google webmaster seems to recommend that)
Taking my example into consideration, does the advise that "why bother with WPMU when you can do it with a single WP" still stand?