with the wide Nitto Albatross bars used by Rivendell, and ideally on a bike with a long virtual top tube and/or with the handlebars raised high. Otherwise, you may discover that the bar-end shifters will poke you in the knees to the point where it could interfere with pedaling on turns. That is exactly what happened when I tried them on my small sized mixte with narrow Porteur bars.
But while downtube shifters offer elegance and simplicity, removing the second set of cables from the handlebars entirely, most cyclists who are looking for an upright mixte find them difficult to use. In order to shift gears, you have to remove you hand from the handlebars are reach quite a ways down. Particularly if you are sitting upright, this is inconvenient - not to mention beyond the skill level of some cyclists.
As far as aesthetics go, a mixte frame is a challenge to set up, because the twin lateral stays already add a degree of visual complexity to the looks. If you add a cluttered handlebar set-up to that, it can get pretty messy. In vintage photos and in handmade bicycle shows, the cleanest looking mixtes tend to be set up either as single speeds, with hub gearing, or with single chinrings - ensuring that there is, at most, only one shifter to deal with. But in practice, most mixtes today are set up with derailleur gearing and either double or triple chainrings - presenting an aesthetic and functional challenge.