If a certain ability had no clear weaknesses, then it would likely not only be frequently used in combat, but also certainly not be kept hidden during a fight. In a life-and-death battle, anything is possible, and if one were unwilling to use their full strength, then even the most powerful of beings might be defeated by those weaker than themselves.
As such, under normal circumstances, if one really possessed a powerful and useful Ability, any living being would surely use it often after obtaining it. But such Abilities, usually referred to as trump cards, could absolutely not be considered a Hidden Trump Card.
Therefore, based on that situation, since those Ancient Powerful Beings had not used these Abilities in previous fights, and only resorted to them now when there was no certainty of capturing Zhou Tian, one could guess that the enemy's current Abilities, albeit powerful, must have some obvious Weaknesses.
