There are a number of quality parts suppliers and manufacturers these days.. HD, C&S, NH, EE, Stan Chen, Brandan Bunker the ones you mentioned and the list goes on etc.. I think most Smiths end up maybe preferring one particular set of parts over another many times base on their style as well as quality. With today's technology I find every manufacturer make or has pretty much the same quality of parts imo if they are made from solid stock and machined to proper tolerances.
Function should always take first place over form in these choices of course but since the quality of so many are comparable, aesthetics and feel become the driving factors in many's choices as does function. ( such as achieving a high grip on a btgs) some parts look nice and are made well but, are just not desirable or as form functioning as others for this particular purpose, or on a certain style build.. None of them got where they are or got their reputation from selling junk parts.
I've used something from all these manufacturers and tbh I've seen the most improvement out of EB parts in the last couple years since they done away with their buy line quality or cast parts.
S&A have made some improvements in their Magwells and grip safeties going from cast to machines parts.
The list really is nearing endless, but each have their own individual aesthetic prowess as well as functional differences, feel etc.. I find most clients who have sent a build list requesting certain parts, usually mix and match quite a bit. But, I will say this, by building a few of those types of guns you find that some parts just don't play well together.. So taste , style, type of build, functional preferences, individual habits, and some other personal concerns need to be factored in.
I believe a build to be a personal experience, each shooter a bit different, as in grip preference etc.. Does one shine above another or are their many that shine?
In other words I believe a lot of things come into play when choosing parts for a build but, many times Quality as well as aesthetic attractiveness is very subjective and objective both.
No simple answer to this question.. The more parts I use the more I choose my preferences and those vary in different styles if builds.. I am ver much finding what plays well together.
Many parts manufacturers make their own guns as well, some have in the past, but found the selling of parts to be more lucrative, some offer parts others don't, simple as that. But, those who manufacture their guns as well as parts, make or have their parts made to fit and to look good on their particular product, if you find their style attractive and functional then you'll most likely gravitate towards their parts.
Oh and btw many smiths modify the parts they use to make them more personal to their particular builds as well. (Myself Included)