They do OK in noise-normalized tests, which are good for comparing and still relatively easy to perform. They're inadequate for quiet operation though. In fact the huge coolers I am using currently are barely adequate for their respective components - they require clearly audible fan speeds to avoid unhealthily high temperatures under sustained load. Certainly not extremely quiet; I'm not
really happy with it, and I have the best air coolers with some of the best fans and the best thermal compound on the market and I have very carefully tuned fan profiles. Water cooling would offer slightly better cooling performance, but it has other issues - mainly pump and motor noise - plus it's rather expensive (more than 10x the cost of air cooling).
You can of course make an extremely quiet SFF build, just not with an upper-midrange CPU and a highend GPU. With the same components you can make a decently quiet mid-tower desktop, like I have.
("Extremely quiet", "virtually noiseless" and so on are pet-peeve phrases of mine - I'm always assuming that marketing people are half-deaf because they keep referring to stuff emitting 20 dBa or more like this.)