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, 2001 XT225
2008 Vino 125, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch Newport moped