It's improved a lot since its inception. In normal circumstances with the latest firmware, regen provides much more braking power than TACC requires; it rarely pushes past 60 kWh, let alone needs disc brakes. From an efficiency standpoint, you can do better by hand than what TACC does, but if you're not trying to hypermile, I doubt it'd make much difference.
Hard/disc braking with TACC mostly occurs in situations where speed changes quickly. For instance, if you're in stop and go traffic, then it speeds up to 40 briefly, then stops again, TACC will accelerate hard and brake hard to react to traffic. A human might better see the larger context of the road, and allow more space to accumulate in front knowing things will slow down ahead and the gap will close.