In my experiments, having an external high resolution ADC, vs the benefits, there is no real advantage.
Once I decided to use the SAMD21, which has a pretty decent and usable 12bit ADC, there was no incentive to add another part and supporting circuitry to the BOM.
Plus, even a good 16bit ADC is pretty expensive.
People may not realize but any extra parts (even passives) add up not just a multiplier component cost, but also other complications, all the way from sourcing, to assembly - extra room in pick and place, to testing etc.