seems like some vendors only advertise in Shotgun News. and it seems like they are your one stop parts sources, got anything you could want.
but there is an absence of detail in the ads. you know you get a BCG, but not who makes the BCG
so, would you buy a complete upper from M&A sales, Model 1 parts, CDNN or J&G Sales?
or skip that and stick to Brownells, PSA & BCM?
Is this about Shotgun News’s ads or about who one would be inclined to buy an upper from? Not sure what the focus is.
As far as upper choice… what is your goal/mission of the rifle? Plinker? Duty? HD? Long range? Vegan Zombie Nazis? Mission drives the gear, end of discussion.
Lastly, I would not personally rank Brownells(Do they even make complete uppers) and PSA and BCM the same.
Beyond that, if you are curious about a particular manufacturer then do some searching, for some of the brands mentioned there is a lot of data out there. Others, not so much, for somewhat obvious reasons that I believe you are alluding to OP.
BCM advertises in SGN 
Usually there isn’t a lot of difference in small parts from one company to the next, with the big variation in quality/importance being the barrel and bcg. If I was at a gun show and needed some small part I would probably buy from any of them, but anything that involved the barrel, bcg, or mail order can likely be done better and as cheap, if not cheaper, by going with a company with a history of selling good quality items.
I have read claims that M&A and Model 1 are connected in some way. Based on personal experience I would not go with either for a complete upper, although I think they do occasionally sell items listing the maker so that would be the only caveat.
With any of them there will be occasions when they sell contract over runs(picked up a new Colt M16A2 barrel from CDNN for $99 years ago) so on those occasions they can give a lot of bang for the buck.
I have actually had good luck with J&T/Double Star in the past, but considering Colt 6920/6720’s are currently selling for about the same price as their carbine copies it seems a bit silly to go that route.