What you are calling an "ADC" isn't just that - in fact, the mic input also has to provide a low-noise bias supply to power the electret capsule, and audio ICs will also provide a preamplifier stage with several different gains being software-selectable (e.g. 0 / +10 / +20 / +30 dB). Only then does the signal end up at an analog multiplexer to make its way to the actual ADC, which these days should generally be capable of something at least approaching "CD quality", way better than what you need. I've heard some dubious USB microphone ADCs but onboard chips have generally be fine, maybe with some attention paid to your choice of sample rate (48 kHz is generally fine, 44.1 may prove more problematic).
As I said previously, both the bias supplies and input preamps tend to have their limits in terms of noise, so for best performance turning down input gain and driving the input from an external preamp tends to be advisable, hence my suggestions to this effect. This should not generally be necessary for speech recognition, but would be useful e.g. when recording voiceovers. While noise reduction can improve the subjective result substantially if necessary and applied carefully, I would prefer having to do little more than some minimal noise gating.
The other (often bigger) issue for high-quality recording or other "serious audio" is that the mic input on a computer isn't compatible with stage/studio mics (which have balanced connections). And pro studio condenser mics need 48V phantom power.
Thanks to the E1DA Cosmos ADC I now know a little about ADCs. The OP asked 'whether motherboard ADCs such as in this NUC are now good enough'. I really want to know, and I think every remote worker who uses VoIP wants to know the answer.
So the microphone quality and placement is 90%; 10% remaining for the ADC (and yet if the ADC is terrible, no amount of microphone greatness is going to result in good sound) so I would suggest 75% for microphone and 25% for ADC.
So for plug in power mics, a low-noise
bias supply is necessary, and then the ADC needs a pre-amp (if built-in version is noisy then get external microphone preamp), and then there's an
analog multiplexer, and then an actual ADC which should capture CD 96dB. And we are all familiar with Realtek 'blue crab' sound which historically generally favours 48kHz playback and recording (indeed many laptops go so far as to turn off 44.1kHz support). We're assuming this motherboard ADC is Realtek-branded.
If it's a good Realtek chip, then the ADC DR might be around 110 dB, but if it's average, 90-100 dB is common. But ADC quality doesn't matter if it's fed with noise, so it's the bias supply and pre-amp along with a high quality mic that matters.
The average person who isn't involved in digital production does not own studio equipment. The most they could have is an IC Recorder from years back, or a PCM field recorder, various laptops and phones, some interfaces, a tape recorder, 3.5 mm mics, usb mics, headset mics and similar stuff.
For consumers, Sony still sells old stock 3.5 mm jack mics from the 80s and 90s used for tape recorders and cut off at 15 kHz, while audio-technica sells either 3.5 mm or USB (unknown quality ADC inbuilt) or XLR etc. None of these appear to be relevant for today's age. We don't even use USB ports that much. XLR is a bit beyond the average consumer in my imagination. 3.5 mm mic jacks are virtually gone now. Wireless bluetooth transmission of recorded sound tends to sound terrible due to compression and mic positioning (eg at the ears). Nothing here suits the current times without diminishing sound quality.
I read somewhere years ago that perhaps crinacle used the apple dongle's ADC via an adapter, because that ADC is the best among all USB-C dongles. I find this to be true. It's not studio quality, but it supports 24/48 PCM recording in mono and there's not that much noise or 'unrealistic sound' (which all other dongles tend to exhibit). Any headset with a TRRS jack (with a large and wide mic) you get for free when you buy a phone can be used, provided that its microphone sounds alright (eg Apple Earpods, Oppo Earpods copy, Samsung headsets etc) Because it's free, can be positioned close to your mouth, and is Apple iPhone communication quality, for voice recording it's what I now use. I think Apple and Cirrus worked out the right way to do the bias supply, pre-amp and ADC, and also the mini mic capsule of the headset which often sounds terrible.