the alliance does have superior technology.
magic, however, i will leave open for debate
Safer technology. The Alliance does not grasp a certain technology that the Horde does not also understand or use. Plus, the Forsaken have a strong handle on biochemical warfare.
Magic, truthfully, is a tough one, namely because of the endless super-powered-for-no-reason Human Mages. But most of them are either dead or not Alliance at this time, leaving Jaina.
And I don't even know what's happening with Jaina.
The horde has inter-dimensional ships too? When did this happen?