So for RPG Maker MV/MZ JoiPlay is creating a web view and playing through that.
Seems stupidly simple and every game should play correct?
Well no, not exactly. The first issue is the NW.js APIs are actually re-implemented in with Java/c code backing the calls for things like file writes, since NW.js doesn't run on Android natively.
Second, some games (Summer Memories for example) expect a mouse and have controls setup for that. As you move from point A on the screen to point B, the movement calls mouse move hundreds of times allowing the game to track where your mouse is. Using a touch screen, you jump from point to point without those mouse updates, so the game may not play correctly.
Third, there's an issue with image size. WebGL2 defers to the GPU for textures. Most desktop GPUs can handle textures up to 8192x8192 but most Android phones can only handle 4096x4096. If you exceed this limit in either direction, then the image will become a black box. And it isn't a total pixel count, a 1x4097 image will show up as a black box on Android.
Fourth, there are issues with certain plugins and performance. In particular fog overlays seem to slow down rendering to a crawl.
If you look at one of my RGA packages (or the changelog JoiPlay notes) you'll see it really is a zip file with some special extras. In particular there's a config file for JoiPlay and a set of dummy files to prevent Android indexing. I had to reverse engineer things from the export function of JoiPlay to get enough information to start making packages.
Also note, a backup of JoiPlay creates a zip file with a password. I haven't dug into that any further.