ValgallaWolf, even if the ballista isn't immediately destroyed, in a long game with large armies, I prefer to buff all units rather than just one ballista. IMO, attack, defense, luck, and leadership with empathy will provide more benefit than machines. The damage, even from an upgraded ballista, is incomparable to the damage from a strong stack of units.

I only upgrade machines for Druids (and even then, only if they level up quickly enough) for the Chaos-enchanted ballista, and for Orcs for the Siege Attack ability.