My rule of thumb is the following:
- Never patch a patch: when a puncture is too close to another patch so that the patches would overlap or almost, then I toss the tube
- Never patch too close to the valve: the valve makes is a structural anomaly in the butyl that makes the tube, so is a more sensitive area, not to mention all the air input comes from there so it is a less stable area.
- Toss a tube that has more than 8 patches... it is an arbitrary number but it is the one I set. Most of the times, when you reach that many patches, you will bump into one of the first two points,...
Of course, when in trouble, when you have no spare tube, and you have a flat, patch it no matter what. That is a no-brainer.
A properly patched area might be stronger than the intact area since it fixes the hole, but apart from the hole, it adds some thickness to the tube so puncture resistance.
You will (almost) never puncture a patched area: the patch might fail, but you will not puncture the patch.
That downside of the patch is that it does not expand (or just very little) compared to the intact tube (try to inflate a patched tube outside of the tire, you will see the diameter is smaller in patched areas). So an overly patched tube will be less expandable in the tire than an intact one.
This causes the whole physics of the tube inside a tire to be less perfect than it is conceived for; which is why you must not over-patch a tube (and solves the apparent contradiction).