Three words: Hit or miss.
The manufacturing tolerances in Taiwan and China just aren't there. I purchased a set of these not too long ago for a 1989 Mercury Cougar, and noticed two issues right out of the box.
One, the driver's lens was not fully flush with the housing all the way around. Not wanting to force it and break it, I settled for sealing the affected area with clear silicone to keep it from leaking.
Two, while Ford makes do with a one prong retainer and achieves tight, movement-free fitment, the foreigners use double the prongs and the thing rattles like hell after installation. All the prongs in the world don't help if you put the hole that they mount into in the wrong place. A bead of silicone on each of the mounting faces where the retainer on the bolt meets the back of the housing quieted this right up. Now they're aimed and work and look great.
I would have happily ponied up the extra bread for Ford units, but I expect that they're long out of production, as were the ones for my 1994 Taurus SHO.
With the SVT, I ordered the stockers from Tousley Ford and have, of course, been thrilled with their quality.