Can't speak much for any sort of long-term dirt/gravel riding, but the 'few' times i've had to deal with gravel on any of the scoots i've had - it wasn't a bucket of giggles.
The smaller scooter had skinny tyres that were knocked around by the gravel at the low speed, and would get bounced around by the bumps in the road etc.
The mid-size was better in this respect due to the larger and wider tyres, however I found low-speed cornering was scary due to the gravel as you'd feel the tyres lose grip over the loose surface. not to mention when I stopped and put my feet down I almost lost it due to the slight slant of the road and the weight of the bike on a surface that wasn't solid.
The big scoot, definitely not knocked around by the pebbles that were around like the other two, but it still wouldn't feel 'safe' to corner at anything other than upright and at very low speeds.
hmmm.....don't like the idea of it very much.
Ride safe my friend.... :)