Probably any with an M.2 slot, but at best that limits you to
PCI-E x4.
If you want to use an x16 graphics card, then you're probably best not putting yourself on the back foot straight away by considering a form factor with limited model availability in the market and in general no native true support for PCI-E x16 graphics cards.
Its target market is more like that of NUC for digitial signage and similiar or for just bolting on to a VESA plate behind a display.
In short if your happy with onboard graphics go for it. if not, forget it.
In days gone by I always like the PC/104 concept of stackable expansion.
its still a form factor that continues and now with
x16 included, but generally you'll only find it used in a few industrial and mostly defense based applicaitons.
http://pc104.org/