I have the same 8mm set from George, with the adaptors, although being this side of the pond, I'm using 8mm collets and not the adaptors.
A dedicated collet has either 3 or 4 independent fingers depending on the make, which enable maximum gripping power.
The adaptor has three slits, only one of which goes all the way through. (If they all went right through, the thing would fall apart). It therefore fits only where it touches and when tightened, is limited in its movement by two of the slits being only partial and thus attached to their neighbour and moving in tandem, not independently. It relies on the inner circumference reducing as it tightens. However, the inner circumference of the individual segments remains constant and any change is a bend at the point of the partial slits, which is not optimal. The grip will even vary depending on where on the adaptor the collet fingers are pressing.
Mostly it will work, until the day the cut is a bit heavier, or the feed rate a bit faster, when the reduced grip will allow the bit to move when a dedicated collet would have held it.
I must admit that I don't know why this particular 8mm set is being offered by George for a primarily US market, although I was glad to get mine. I only wish he'd offer more 8mm shank stuff ,as he is far cheaper than anything otherwise available over here. There are times when something more rigid than 1/4" but not as big as 1/2" is handy, particularly if I don't want to use the bigger routers.