Because it is based on engine data to compute left and right thrust estimates, it is somewhat vulnerable to an engine failure that takes out the associated sensors. Just when TAC is most needed, it may not be available. For that reason 777 pilots must go through engine out training without TAC just in case.
Maybe not as vulnerable as some might think. Boeing recommends the following possibilities of TAC disengagement :
Engine Separation 100%
Severe Engine Damage 10%
Engine Surge / Stall 1%
But, this sounds good :
On the 787 the equivalent of TAC is provided without such heavy reliance on engine data such that it is no longer necessary to train for an engine-out during takeoff without the ITAC (inertial sensor based TAC).