Well Tia hold back all the time by 99.99% all the time, she can't let her self lost control, if do, no one know how dangerieuse is will become, Is not a Surprise why she let somepony fight for her only if she know this one or a group can win against a treat but she know no one can do it, she got no choise to do it her self and hello big destruction(if she use more of her power she allow her self to use) Luna is only 80% of the power of her sister but she dont have a 1000 of battle and ruling on the land like her sister, her wisdom is not the some as her older sister but she do her best to be a great princess like her sister is be for not be in the shadow of her sister like befor, Luna too fear to lose control but not as the some as her sister, she fear to become nightmare moon again but thanks of the support of her subject and her sister she got less fear and more thanks of twilight and her friends for she trust her self more, Tia her lose control of her power and her subject is what is scare her, in the battle against chrysalis, she a hold back by fear to hurt the pony in the room, is the battle be out side or be no pony around, i pretty sure she will use more force, i dont say she destroy a planet the character in dbz but she is very powerful but wise to know where is time to fight or where is time to not, plus she a ruler, she can't go in battle all the time, if she gone missing, what the pony will react? equestria need thier Alicorn leader's! this is why Tia a make Twilight a Alicorn and Cadance, she know something like this can be possible! Plus Rule equestria all alone and make sure is still in pease is very hard! some fan believe Nightmare moon(luna) will be a better ruler, Sure, she will start a war all the time against all are against her no mader what race, celestia her a be respected by her action, nightmare moon force other to obey her, celestia dont force but try to fond a solution on the problem and peace, go in battle is the last solution she will use if she a no other choice(but she will never happy to do it) Nightmare moon will choice violence over peace! this is why luna hate her self for be nightmare moon, this is why she do her best to be a great ruler she need to be and her subject need. not a tyrant, this is why celestia is see as a figure not only by her subject but her sister too, this is why she want to be great like her but as own way and is the same for cadance and Twilight both want to be the ruler equestria need and be love!