Focal reducers are awesome in theory, but it seems that the Metabones Speedbooster and Speedbooster Ultra series focal reducers make more of the theory a reality than the rest, with the Zhongyi Lens Turbo II trailing close behind.
A perfect focal reducer wouldn't alter any characteristics of the base lens, but any real focal reducer will to some degree. At the very least, the additional surfaces of glass (coated or not) will introduce their own flaring / ghosting characteristics. The original Lens Turbos infamously had some issues with these.
The Metabones reducers are designed to worth with a wide range of focal lengths, with a wide range of exit pupil distances, and really fast base apertures. They were also designed with the sensor stack of their intended systems in mind (Sony E, MFT, etc).
Not all of that can necessarily be said for sure of the ZY LT II, the Viltrox reducers, the Kipon ones, or the "no-name" brands, but we'd really need to investigate patents (and do real-world comparisons / tests, which is difficult to do fairly) to see. I don't recall ever seeing anyone do a comprehensive shootout, but we should be able to dig up a couple one-on-one comparisons if we Google or fish through our bookmarks/history hehe.
Having used a couple ZY LT II reducers personally, I've found that they introduce central SA for already-fast base lenses (f/1.4, f/1.2, etc), but that they perform really well with slower apertures/lenses. Comparisons done in the past on this forum confirm this behavior, where it's hard to tell the difference between a ZY LT II and a Speedbooster once the lens starts getting stopped down. There is also a bit of a greenish/yelow color cast introduced by the LT II, which might be flare-related. Both of my LT II reducers were bought used for around $50 USD a pop, after the focal reducer trend kinda died down.
Speaking of which, now that full-frame MILCs are so accessible, reducers make less sense these days once you consider the cost of a "good" one, but they still have their niche in videography, where people might want or need the extra stop of light, the increased field of view, or to reduce the loss of field of view when going into crop modes that unlock higher framerates on some cameras.
