Edwin Pawlowski writes:

Absolutely. This is the nature of the Bourdon tube mechanism.

Can you quantify "unreliable'? Will they stick in one spot,? Not go
up? Not come down? Read high? Read low? All of the above?

All of the above are typical except perhaps reading high.

The least bit of corrosion from a wet or condensing environment will spoil
the little gears and bearings.

If you ever have a bad one to throw out, tear it apart and have a look.
Looks like the proverbial ladies watch.

Here's a pretty good explanation and diagram:

http://www.tpub.com/content/fc/14104/css/14104_232.htm