Even if towers are counted as units the whiplash should show the crit which it does not and i doubt that the towers are wider than 200 radius. But on the other side i can understand why they won't let the crit be able to procc on units like towers since you could nuke a tower down way too fast. Though i think it should atleast crit on the creeps within the 200 radius of whiplash when you attack on a tower.
One solution could be to just let the skill not loose charges when you attack mechanical/units and save it for enemy_units instead. Dunno how the balance would be though but i don't see it would make any big difference.