Why could it be dangerous if an electrician finds a water pipe in a bathroom with a plastic repair?

I do not know Plumbing code but I will give you an electrical answer.

If there are any other metal pipes connected to your fixture, in this case a lavatory, commode, shower or tub, it changes nothing. If there are no other metal pipes connected, you electrically isolate your fixture from a grounding source. This means that any electrical source that comes into contact with the fixture or the water it contains will not cause any current to flow.

If this were to happen, this is actually a safer installation than being grounded. If a hair dryer or similar appliance gets dropped into the water, and if the fixture is truly isolated, there is no risk of electrocution. I WOULD NOT test this theory.

But if the fixture is grounded it will facilitate the operation of the breaker that feeds a circuit that comes into contact with the fixture or the water in it.

All this being true, all receptacle (outlet) circuits that feed a bathroom are required to be GFCI protected, which is a much safer protection device than the breaker.