Oops. Guess I misunderstood you.
"Boutique" to me seems to be more about the size of the company and their marketing strategy and not anything to do with quality.
I think that in many areas, just playing the odds, a new boutique brand is likely to have inferior engineering to something from a larger and more established company. This causes an, IMO justified, waryness of boutique brands. OTOH, some people seem to forget about how tiny and boutique a brand is once an independent source verifies its performance.
As I mentioned above, "boutique" used to be a good thing, whereas
@BillG thinks it's bad. How representative his is, I don't know.
I agree entirely that a lot of it has to do with branding. Some small businesses want to look big, which they can do with effective marketing and is very common with audio companies, and some big companies want to look small. A lot of the latter has to do with exclusivity, think the Esoteric brand of TEAC, Lexus brand of Toyota, etc.
The advantage in UK and Europe is that corporate financial, directors and ownership information is publicly available online, whereas in the USA it is only available for public corporations. So here you can get some sort of idea who you are dealing with.
Still, you can have larger companies run into problems with OEM components, like PS Audio who used Oppo transports (closed the business) and Leica who used Kodak sensors (went bust).
I have a boutique turntable, one of only about 50 made. It was a side project by a design/fabrication engineering company with incredibly expensive machinery and is better made than just about anything else out there. They weren't interested in the time and money to market it, but still do lots of design and fabrication for other audio companies. Because they have the in-house manufacturing capability, it was a lot cheaper than competitor products. They also sensibly bought in industry-standard motors and belts. I love it, which is good as it has $0 resale value.
I think corporate profile is an important issue often overlooked, much more complicated than the
@BillG generalisation I picked up on.