We do in fact mix fonts transparently (unlike some other platforms). That was always fontconfig's main selling point.
However, proliferation of fonts that only cover a few unicode blocks is user-unfriendly, and quickly overcomes the font management capabilities of humans. (it's about the same phenomenon as toolkip proliferation before GNOME and KDE started producing well-integrated consistent wholes)
Fonts with wide unicode support are still the best solution if done right. And since this article is about free fonts, anyone can step up to improve part of them if he feels they're not good enough.