I don't know but I just ordered one of the Black Diamond 6.8 SPC uppers from Midway USA. I have used their products including suppressors and have had very good luck with them.
I think they are viewed as middle of the road as to design and quality. I've got one of their free float quad rails and flip up iron sights.
The sights are pretty decent the only thing some people don't like about the front gas block/flip up sight is it is not pinned, but rather just secured with a pinch bolt arrangement. For me meh, not a big deal, that thing isn't going anywhere, and you can take it off and on without removing the flash hider.
According to their website, they do not offer a non chrome lined barrel. Is it an older one?
I have the 20" diamond fluted barrel. Fit and finish is nice. Has M4 feed ramps if thats your thing. I cant attest to how well it shoots as I have yet to top it with glass.
Are you speaking of the complete upper or just the bare receiver? If it's just the naked receiver, I just used one on my first build, it all came out very well and fits very well to the S&W lower. As far as other YHM parts, I've got a YHM gas block/flip up front sight and fixed rear sight. The only problem with the gas block is it's size, there's not much surface area on it, so it heats up quick.
I'm building a rifle at this very moment, and have a Yankee-Hill upper assembly (upper receiver with dust door and forward assist), gas block/flip-up sight, and their smooth free-float hand guard (with end-cap). In fact, I have the same gas block/sight on my M&P15 MOE.
It may be considered middle-of-the road, but it all fits as advertised, and the upper mates up nice and tight with my Spike's lower.
I like spikes tactical.. But don't risk ordering straight from them. I have had problems, my friends have had problems, and ive met strangers that have had problems with them(one a nightmare story about a piston upper he got from spikes). Regardless I have called them and ordered parts they told me were "In stock" and waited over a month to receive that item. Same with a yhm rail my buddy ordered from them.
Again I love their products and I have 5 lowers by spikes but will not order straight from them.
As far as the yhm I have had good consistent luck with all their products I own including rails, suppressor, sights and I would recommend them but I do think you can find better stuff out there for about the same prices.