Vehicles taller than 2.2 metres (slightly over 7 feet) had to find another route around the TEB. When the TEB turned a corner, every vehicle underneath had to wait for the manoeuvre to complete before going around the corner themselves. And no one had really worked out how to solve the issue of cars driving into the "legs" of the bus.
These issues could be solved if driving becomes fully automated. Tall vehicles could be programmed to avoid TEB routes and turning maneuvers could be efficiently co-ordinated by computers. In the world we currently live in though, the TEB is a terrible idea.