How reliable/accurate are oil level sensors?

I used to have a Mk3 Escort which had a sensor on the dipstick - a bit of a pain as you needed to disconnect the connector to check the oil the
old fashioned way. I can't recall if it gave a level or just a low warning.
Our new Fiat MH, has a sensor somewhere, not on the dipstick, at least there is no connector so I assume not. It gives a level reading, if you scroll to the right menu. My question is, how reliable are these? Unlike the old Ford ones, which got a wipe when checked the old fashioned way, what stops gunge covering the sensor?
I'll still be using the old fashioned way, I'm just curious.
(I am referring to level, not pressure, sensors.)
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