I have a ZhongYi focal reducer, but no others to compare with. I have included crops from photos taken with it at the end of this post. Maybe others can post crops from a series of photos taken with other focal reducers for you to compare.
Overall, I have been unhappy with results I've gotten from adapted 35mm lenses. The color and sharpness are not as good as I remember getting on film. Modern lenses, especially if they have OIS, will easily outperform old film lenses. (There are threads on forums with people taking amazing photos with adapted lenses. I'm not at their level.)
If I had a do-over, I would not bother with the focal reducer or lens adapters. I would stick with image-stabilized, native-format lenses. If you must put your old lenses to use, I'd suggest playing with plain adapters first (if you haven't already). If you're not happy with the results you get from those, it's unlikely you'll like what you get with a focal reducer.
The only lens I have that is close to worth using with an adapter is an MC Zenitar-M2,8/16 fish-eye. It's reasonably sharp, has good color, and provides a different perspective from ordinary rectilinear lenses. I do have a few issues though (mainly my own fault):
It is difficult to focus because everything is so tiny. What I end up seeing on my monitor at home isn't necessarily what I thought I'd captured. Somehow, I do better focusing by wire on modern lenses.
There is no image stabilization. Even though the lens is "fast", image stabilization makes a huge difference. (I've gotten pixel-peeping sharp images, handheld, at 230mm F6.7 at 1/15". That's about 4 stops worth with the 1/focal-length rule.)
With the focal reducer, everything is soft-focused unless it's stopped down. In the images below, it looks good between F5.6 and F11.
Here are some 100% crops from photos taken on a FujiFilm X-E2 on a tripod. I'm showing the most in-focus section within the central region, which isn't necessarily what I thought I was focusing on. Here is a set taken with the focal reducer at (F2.8, F4, F5.6, F8, F11, F16, and F22).
Here is a set of images taken with a plain adapter, without the focal reducer.