Not that I really want to get into the middle of this, but really both sides are right.
Vintage scooter riders do tend to rely more on their rear brake than modern scooter or motorcycle riders would, but I suspect that's largely as a result of preference as well as because of serious suspension dive compared to more modern scooters; of course, there's also the relatively poor performance from the front brake, but often that's matched by the lousy rear one, too, as starreem himself has pointed out.
According to the physics of the matter -- and assuming same brake size -- the front
should be more effective because of weight transfer to the front. Given the same amount of braking force applied to both front and rear, the front should be less likely to lock up than the rear (which has "lightened"), and hence should be considered more effective (since you should be able to actually apply more braking force to it before locking the wheel).
The reality is, however, that many older scooter drivers prefer to rely more on the rear. Sometimes the front really is less effective, but it's not because of physics, but the state of the repair of the scooter in question. It might have a warped front drum, or one can't get a lot of force on the lever, and even at full pull it doesn't seem to do as much as the rear. I think that's from poor maintenance or settings, however.
I have a '78 Super 150 with 8" wheels and drums. Brakes don't get a lot less effective than that.
That said, there's plenty of stopping power considering the light weight of the bike, relatively low top speed (about 85km/h), and if one rides with any amount of sense and anticipation. I've had a few "oh sh*t" moments, too, though, and have been surprised at the overall amount of braking force available in an emergency. (I can easily lock the rear, though have never done it to the front, thankfully).
All that said, I've experienced large changes in the effectiveness of the front brake simply by making some adjustments and repairs. There were times when I could do a full front brake grab and wouldn't get that much stopping power, and definitely relied more on the rear. After some adjustments, including a new front drum hub, checking the pads, putting on a new return spring, and installing a new cable and teflon-lined outer housing, I had MUCH better front braking power. Now it's definitely better than the rear, and I rely on it a lot. I always start with the rear since it lights the brake lamp, but by the end of a stop have a significant amount of front brake applied too, and if I need to stop quickly it definitely does a lot more of the work. Your mileage may vary....