Everything appears to work, but the I have to call the overall experience quite dysfunctional. Documentation is poor, leaves out large portions of information that the programmer seems to think is "too obvious" to tell us (or something, I'm guessing. Point is there's steps missing). Best to start with the presets and reverse engineer them to make your controller work (assuming of course that one of the presets doesn't suit your needs simply as-is).
Didn't play well with my retro controller. I used JoyToKey to remap it to different keyboard keys and set the overlay map as a keyboard overlay with the retro controller images. Speaking of which, the background image, the base controller, can't be included in either the OBS plugin or the (more importantly) the provided web tool to generate the .json file for your controller, for what reason I have yet to fathom. You just have to guess where the controller buttons should land and push them a few pixels at a time this way, a few more that way, until they sit properly on the base image. The image swap on the button presses is also a lot more cryptic than it needs to be. He's got this 3 pixel gap thing going on that seems to work fine in the videos but somehow it doesn't work that way when setting up your own controllers.
[sigh] ... all that said... if you can get through all those roadblocks, or, if one of the presets suits your needs, it appears to work just fine. I am satisfied with the final result, at least. It has not broken so far... knock on wood.