Bad fuel gauges?

Hi all,
A recurring theme seems to be that one should should not rely in fuel
gauges. I can understand that from a safety point of view (I always
dip the tank before start), but I get the feeling that they are
considered just plain inaccurate. Why is that, every car I've driven
has a fuel gauge that seems accurate. Am I missing something?

A connected point is that I was taught that if you start to worry
about low fuel you switch to the lowest tank (2 tanks) and note the
time. When the engine splutters you know what time/range you have left
to find a good landing spot (after switching tanks). But if a gauge
can't be trusted is that the best thing to do?

Cheers