mwha, imho bonusses to unbinding shouldn't be needed in 99% of the cases, even if +1 to casting is fairly common. If we take us as an example we can get +2 to cast with an astrolith & a slann with the right constalation. However, this requires 2 different models, and some luck. Which imho makes it acceptable. There's a fairly decent cost involved by having to field 2 models, it relies on luck with the constelation, the models aren't that difficult to kill. So there's enough to fiddle around with to call that balanced.
However, characters like Nagash that just have a 3+ bonus for merely existing are rather problematic. Against those a better mechanic than just "murder the thing" is needed. Though in fairness Nagash is just Always going to be flat out broken with regards to magic as long as he keeps his ridiculous bonusses. Being able to cast 9 times when the second highest is 4 and the average is 1 is just silly, just like having a 3+ bonus without needing to work for it is rather stupid. Nagash just kinda needs to be reigned in if they want magic to actually get somewhere without constantly needing bandaids like the rule of one to stop him from singlehandedly wiping out armies in 1 turn.
I can't think of any good mechanic for it though. Just making anti-wizard units that have a +1 to unbinding will result in either an arms race to the bottom, or eventually a stalemate where both sides just Always have say a +3 bonus. Plus if the anti-wizards become common enough it'l make wizard-based armies even more annoying to actually use, the rule of 1 already can make it a pain at times. Something like a anti-astrolith would be obnoxious as hell as a wizard to deal with since there's relativly little the wizard can do if the anti-astrolith just hangs in the back. Maybe a unit that gives a +1 to unbinding if it is very close to the wizard. At least then they first have to catch the wizard as well so the wizard has at least some time to strike first?