I've seen a cheap bubble balancer and then seen the same wheel put on a Coates bubble balancer to compare. I'd trust the Coates and they weren't registering the same.
Not just a stiff motocross tire. A stiff moto-
cross tire with two street bike tire treads inside of it and several hundred sharpened screws all around it. They are quite heavy too. Especially hard to mount 19" tires because they already have a narrower side wall. I know guys that have ice raced for 20 years or more and even the best tire mounters struggle with some of them.