[sokoloff]

The sink supply connections (usually from a shutoff valve) are compression threads (a parallel/straight thread).

If the valve itself is threaded on, that inlet might be NPT (or might be compression), but the valve outlet will be compression, which uses a tapered ferrule to provide the seal and straight threads.

[mindslight]

The faucets I've seen are generally NPS where the supply lines attach to the faucet, often with the retaining nuts (or whatever they're called) using the same threads and going on first.

To answer GGP's question, what's difficult about faucets is getting up behind the sink, with your back straddling the corner of the cabinet, reaching up with some sort of basin wrench that inevitably won't grip those bespoke retaining nuts well. Especially removing the old faucet where the retaining nuts are a bit seized.

[blincoln]

> The sink supply connections (usually from a shutoff valve) are compression threads

I've seen both compression and NPT. I only have three faucet-replacement data points to work from, but I haven't developed a feeling for why manufacturers will choose NPT in some cases.

Compression fittings are pretty straightforward, IMO. No complaints from me about those.