Whatever doesn't kill you makes you stronger

I think the original quote was "that which does not kill us makes us stronger" from Friedrich Nietzsche, although I haven't bothered to check this. This quote reminds me of Spartan ideology in which the youth were made to suffer in order to make them good soliders. I've tried to think of a few examples to help from an opinion on the validity of this statement:

Physical suffering -> physical strength

Scar tissue is stronger than normal skin.

Physical suffering -> physical weakness

If cut someone's achilles tendon they will never walk again.

Physical suffering -> mental strength

Even if you cut someone's achilles tendon, the forced change of lifestyle may make the person wiser.

Mental suffering -> mental strength

A person may be greive when one of their parents dies, but this experience may make them realise the value of their relationship with their remaining parent and make more of it.

Mental suffering -> mental weakness

The world is, unfortunately, littered with examples: the bullied become bullies; the marginalised become terrorists; those that once loved become bitter and twisted. Although it could be argued that some of these responses are a form of strength in that they protect the person from further damage, I consider mental strength to be the ability to make the right decisions even when they are difficult.

From the examples I have thought of, I can only conclude that this statement (at least in its literal interpretation) is wrong. Suffering can make you stronger, but it seems it can also weaken you. There is probably a threshold for every type of suffering, for every person, at every time above which they will be damaged, but below which they will be strengthened.