Fundamentally, the reason they're especially accurate is that most of the usual sources of error in digital–analog conversion just don't exist in a 1-bit ADC. INL? Zero. DNL? Zero. Nonmonotonicity? Please.
Clover, by contrast, is interesting to me for a different reason: a lot of ANN inference stuff seems to work fine at 8 bits of precision. It'll be interesting to see if those results can extend to 4 bits.
Now I need to study up on the area of computation that clover lives in.
I dare say that the most common AD converters are simply delta-sigma converters, especially if 1MHz conversions is fast enough for your tasks.
To get into the 100MHz or above speed (ie: Oscilloscopes, Software defined Radios, etc. etc.) you need to use other techniques.
That gets back to my original point. The spectrum analyzer went into the gigahertz range. I don't think it was delta sigma. It was some multiple sample thing that only worked in the frequency domain.
I distinctly remember that Delta-Sigma was a slow-type of ADC. And I guess I confused it with the one in the ATMega328p.
The ATMega's ADC is pretty slow, like 15ksps at full precision, so I can see why you'd think that!
They require higher clock frequencies and aren't that accurate (though, as you mentioned, some other sources of inaccuracy are simply nonexistent)