Honestly, the precedent im thinking of is the various Mega forms thatve been banned in OU, like Blaziken (though thats mostly because hot chicken was also banned), Metagross the unfairly banned (they seriously said something along the lines of "it can run ten moves at once" because apparently having multiple sets was too much?) and Sableye the straggler, among probably a couple others.
Now, if its the only thing Terapagos can tera into, then yeah id say its unequivocally complex because youre saying "cant terastalize if its terapagos"