I can tell you what the "problem" with TBN is, it isn't free. It is at best dps neutral and at worst a dps loss.
The 3000 mp you're spending on TBN would normally be spent on an Edge and in theory yes, for those 3000 mp you get a 25% shield and an Edge...but only if the shield actually breaks.
This means TBN is limited to attacks that can actually break the shield, not the biggest problem since you just need to know what breaks it and what doesn't. Calling it "free" is however just flat out not true because if it were you could just use it on cooldown. The other issue is that you basically get less use out of TBN the better your gear becomes since there will be less attacks that can actually break it.
All in all I wouldn't consider it a big "problem", it's just a very strong shield that costs you dps if you use it wrong.
