After trying to live with a folding bike, here are my observations. Most, (all) foldy bikes have a feel different from a regular road or MTB. so the feeling is never going to be the same. There might be some flex from the handlebars, they could be slow or feel as though they are going to fall over.... So I think to a large extent it would be best to put their handling characteristics out of the way.
I think a true measure of a folding bike is:
how it folds,
how easy it is to walk up the stairs with it,
how easy it is to put it in the boot of the car,
how easy to walk 10 meters with the bike in its completely folded state.
(or how easy it is to fulfil your portability needs)
Similarly a downhill mtb with 7" of suspension is not going to work so well as a regular street commuter, but in the right environment its horses for courses