If it were completely unreliable you would never be accurate even when reporting your own opinions, except perhaps by chance.

I didn't say it was completely unreliable. I said it was completely useless.

As the saying goes, a stopped clock is right twice a day, while a working clock is unlikely to be accurate within the limits of measurement. However, the stopped clock is extraordinarily reliable - reliably useless, because it really tells us nothing at all about what time it is. The working clock may not either, but it could at least potentially be a useful guide.