If these are the same thing as E Z lube, the system can work if done properly. There is a video on the Dexter site simulating the principle behind the system. Yes, the biggest risk is blowing out the rear seal which in turn would contaminate the brakes.
If you elect to try it, its best to do when its warm out and even better when the hubs are warm as in just having been on the road. Lay the grease gun in the sun, too so help soften the grease. Lift the rig or each wheel off the ground and have someone turn it the entire time you are slowly pumping grease into the hub. Continue to this until you see clean grease coming out of the front.
As others have said, its best to pack bearings the old fashioned way.