
I'm writing this from the point of view of someone who loves mixtes. I like the way they look. I like the way they ride. I find it incredibly convenient not to have to throw my leg over the back of the bike every time I mount/dismount, particularly when I'm carrying stuff on the back or wearing a skirt. I really don't miss the top tube when I've ended up stopping suddenly due to traffic or in rough terrain.
I don't have to explain to you (or this group) why I like Rivendells. I have/had 2 vintage mixtes and much as I liked both of them, the Betty Foy is a far superior bike. I spent all summer and fall bombing around on 47 mm tires and had too much fun. I'd love to do several days on a rails-to-trails, mixed terrain type trip with it like that. Right now, as the photo shows, it's time for fenders.
I'm trying to think of something to say for the "what don't you love" part of your question. You do have to use an adapter bar-thingy to put it on a car rack, but that really isn't any trouble. The bar lives in our station wagon, and it takes seconds to put on.
I can only imagine how great Rivendell could make a custom "mountain mixte" for you. GO FOR IT!