The only people that use under arm crutches here in Europe are those who have hand or other injuries that mean that they cannot use elbow ones.
The elbow crutches are the norm over here and can help with normal gait much better than under arm ones. I've used both and know which ones I would choose. We are rarely told to use just one crutch the normal use is two at a time (as with under arm ones) then we are taught to use them alternately - left crutch right leg, etc. Normally they are then shortened so that they are not the primary support when the OS says FWB. Then you carry them in case you need it then do away with them. As they are lighter it is easier to go up and down stairs using just one and holding the other one.
Sue