There are very few which are suitable for integration into other products - I currently build a scientific instrument that needs a fairly powerful SBC to run. Intel NUCs were well supported and documented: all of their firmware was updatable on linux without any issues, they had data sheets with power specs so you could run them off DC predictably, and you could buy boards without a case. There are plenty of small NUC shaped mini-PCs but few that are suitable for integration (at the price point intel was at).

Cheap Chinese mini PCs just aren't well documented and don't have predictable supply.