There aren't very many actual "makers" of AR lowers; while not a tiny list, it is certainly dwarfed by the number of "brand names" that sell lowers.
And the are two steps to the making process: the raw forging is made, and then it is machined to final dimensions. As long as the forging comes from a reasonably well known forge, the quality of the lower is up to the machining firm. Dimensional tolerances should be pretty good on a lower made by any reputable machining company, but ther can be differences that a purist might find acceptable or not, such as how the magazine well is made (cut versus broached).
From a "fit and finish" standpoint, I've been quite impressed by the Stag guns I've been able to handle, and I've never heard anything bad about Spike's receivers.