I have 2 125cc scooters (Yamaha Zuma and Vino) To me the main advantage to an underbone type bike is it's manual transmission. My scooters won't climb worth a s**t because of the very narrow gear range of the CVT. They do not have low enough gearing to climb mountains, while a manual transmission bike can be geared down to climb anything. Honda realized the need for super low gearing with their Trail 70/90/110 with the dual range transmission. Since a CVT is so limited in gear range, they are geared for top speed, not climbing.
2002 Vulcan 750 (now being slowly reassembled) 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
1980 Puch moped