Hi there, we have 5 TIUs with the TVS and beefy clamp in parallel. In 3 years we haven't had a single channel degrade or die except, one TIU that burnt down when the building contactor failed and put 400V RMS to the single-phase outlet. It does seem pretty rugged.
The TVS turns on very fast (like 100ns-1us) so it protects you from very fast transient events. The only issue is if you have continuous voltage overshoot it will eventually go into non-reversible breakdown. The 1n4148 clamp is very slow to turn on (like 5us range) and won't protect from the like (1-1000ns) events, but can handle over-voltage pretty much indefinitely. So if a continuous overshoot occurs the TVS clamps quickly and then hands the shunting current over to the 1n4148 after a few us so it doesn't break. That's why the 1n4148 stack needs a slightly lower threshold than the TVS... so in the steady state all the current in in the 1n4148, not the TVS.
