Buck lost the contract on M9's because they could not produce an "acceptable" product as cheap as Lan-Cay (and later Ontario) could.
Buck used a completely different process than Lan-Cay and Ontario. The difference is obvious in the blades and even the guards. Buck machined their blades, and Lan-Cay and Ontario purchased new laser-cutting equipment to laser-cut the blades...it's cheaper and faster that way. Lan-Cay and Ontario also didn't bother with the blood groove, so they're a little heavier and somewhat less refined.
The only Ontario I've ever handled is the one I own. But the laser leaves rough places on the sides of the blade, and the blade on mine is warped....I can only assume from the heat. I don't know how common this is. While it would work fine as a bayo, and it's solidly assembled, it doesn't make nearly as nice a knife.
Also, the older Phrobis/Buck scabbards are nicer in that they have more straps, a Beretta mag pouch in front, and a sharpening stone in the rear of the scabbard. The newer scabbards don't have the integrated sharpening stone or the pouch...more cost cutting.