Very late to the party, I think tera should not be banned.
I think tera is a very fun mechanic, it enables so many teams to become more viable, getting better typing on the support like zygarde to cover the weakness of the team. In battle, it's just more mechanics to play mind game with your opponent and I'm all for it. At most you lose one mon from the tera's sudden switch up (and I think it would be mostly the player's fault for not anticipating the possible tera) & have to deal with a mon with a different typing. Rarely would tera be victory deciding. I think learning to use tera on your own mons at the right time and anticipating opponent's tera types should be learned, this makes the game be more dynamic.
It's my first time trying to say something intelligent on a forum. AHHHH! Did I do good?