Yes, with USBGX, you can boot Uneek on usb only, you don't need SD to boot Uneek.
On Wii, USBLoaderGX is using the same boot method than the vwii (without bootmii), so you don't need bootmii folder nor SD card to boot Uneek.
ahh, just a thing I'd like to add :
if you are using neek (sneek or uneek) for the first time, do not put di.bin in the folder just yet !
you can generate it if you want, but either rename, (re)move it from sneek folder.
first, be sure everything works without di.bin file.
di (disc interface) is only used to mount disc based games on the official "disc channel" on neek's system menu.
but, it's often the reason neek is not booting, and not neek being not properly setup. so, first, setup neek and be sure it works to prevent not understanding whether you properly setup neek or not.
when neek works, try putting di.bin back in that folder and see if neek still boots.
if all works, then it's good
if it back screen at boot, then di is the problem, especially on neek (not neek2o) because it requires a specific additional setup : an extracted disc ! if there is no game on your sd or usb, di freezes.