The doses used for nortriptyline are much lower (sometimes only 10%) than the antidepressant doses.

At the low end therefore there is not much risk of overdosing unless one takes a whole bottle of them.

The tricyclics when used for depression were used 100mg + a day (up to 200-300mg a day). At that dose, and when mixed with other drugs that affect liver enzyme metabolism, THEN, an QT event may be triggered.
When tricyclics first came out on the market for depression, these liver enzymes had NOT been discovered yet. And suicide attempts with them were often fatal because they affect the heart rhythm in high doses.

Today there is alot of research on QT events, because this was happening with a common antihistamine called Seldane --which was taken off the market. Seldane used with erythromycin or antifungal drugs caused the heart to stop.

There is a research organization now for this topic:
https://www.crediblemeds.org/

Now, the issue with Cymbalta is different. This drug is not used in tiny doses for PN.... it is used in HIGH doses right from the beginning. So the liver toxicity is much much higher in potential than with nortriptyline.(or amitriptyline). Cymbalta caused some deaths even during its clinical trials. None of the other SSRI/SNRI drugs have that history either. People who drink or who already have liver damage, are most at risk for liver damage with Cymbalta. But damage has occurred in patients who do not drink alcohol at all.

Therefore Cymbalta is not for everyone.