Underbone and Scooter are for the most part the same. The only differences are the tire size, riding position and comfort they provide. Underbones are somewhat unique to Asia, I think. Scooters have features like bigger compartments to store things and are generally more comfortable to ride on. Underbones are more flexible. An underbone can be personalized to suite its owner. There are lots of accessories available for them.

In regard to accidents on big and small bikes. The reason why there are more accidents in the Philippines that involve small bikes is because roughly 80% of the total motorcycle population are small bikes and only a few are on big bikes. If it were the other way around big bikes will be involved in more accidents. The same logic applies to four wheeled vehicles like cars and trucks.