Is GNB bad or is TBN too good?

[Kaedan]

could it be, you're using your TBN a bit too slow?

This, most likely.

It takes a bit of getting used to the timing of TBN. There's a bit of a "spin up" time for the shield to actually activate, so if you activate it when the tankbuster cast bar is over 50%, there's a good chance it won't be "up" in time for the tankbuster.