Well lateral balance needs to be close, mine isnt perfect and none of my planes have wanted to snap provided the following.
The elevators are setup with perfectly equal throws and they get there at the same time. If you have dual elevator servos and one has more deflection that than the other you will snap out of a wall OR
One elevator gets there faster than the other. Sometimes setups are such that the elevators end up in the same place but one gets there faster than the other. You control linkage geometry needs to be identical on both sides. I dont think that slight variations in servo speed matter enough to make as big of a differnce as setup does. OR
A weaker servo on one side than the other. Both servos have to hold equally under the stress of the maneuver. Regardless of what your servos do under a static load sitting on the table, if one of them gives in to the pushback during the force of a wall, your throws are not equal and the effect is the same as if your throws arent matched on the table. OR
The plane is too heavy, if the plane tends to snap at unwanted times during normal flight or when landing, its certainly going to want to snap in a wall. The plane needs to have a low wingload.