My experience with streamers have all been custom builds, learning from 'expert' members over at Audiophlestyle forum. Each has had unique sound qualities that were easily heard. Over time I discovered power supplies most greatly impacted sound quality, quality of parts populating motherboards, operating systems and music players also affect sound quality in my setups.
Optimized interfaces also critically important for both streamers and dacs, some of these streamers not optimizing usb and I2S interfaces to their fullest, this means highest quality clocking and power supplies for those clocks. Many are adding a second streamer to these off the shelf streamers. These second streamers act as endpoints only, therefore, they contain motherboards and operating systems designed for a single purpose, which is to act as that endpoint, point of this is to minimize self generated noise, provide lowest possible latency. The second purpose of these steamers is to provide a more optimal usb or I2S interface, this means better clocks and power supplies than provided by many off the shelf streamers.
Unless all these off the shelf streamers using very similar power supplies, parts I find it hard to believe they all sound that similar. But then perhaps they do, evidence of that in this thread. I look at something like the highest end Taiko servers, based on reviews these are game changers. Streamers at this level take every concern I've listed above, and there are more, and concentrate a level of engineering such that self generated noise and immunity from external noise creates a nearly unlimited noise floor which means higher resolution. They also have proprietary and optimal interfaces which surpass commonly found usb and I2S interfaces.