Quickly, I found, the pb4 software run on koa board is not very perfect, just like the game 'sunset riders ', The game screen is not correct, the aspect ratio is not correct, so if you want a better game experience, you'd better get a pb4 boards.
You're right, when running the PB4 SD card in a KoA board some games are unplayable due to it messing the resolution up (games appear squashed into a thin ribbon about 1/5th of the screen wide). Neo and CPS1/2/3 games don't seem to be affected. Still you're adding hundreds of games for the price of a microSD card so I can't complain much.
Manual claims they aren't but the 5th and 6th buttons are wired in and working, no games use them but they're even present in the test menu and work fine when using the PB4 software. Screen tearing or sound issues in games seem to me to be the exactly the same as how they run in horizontal display on the PB4. I don't have a lot of shmup experience so I can't compare them to the original boards.
It has the same graphics smoothing/filter applied as the PB4 and also handles credits the same weird way. Free play jams in 9 credits every time you hit start, but even with it off it doesn't credit properly. You can coin up a dozen times and the actual game won't register it but the frontend will, then at some random point (or when you hold start to bring up the exit menu) it will up you by 1 single credit. Hardly a dealbreaker but a strange annoyance.
Maybe it has something to do with the wonky credit settings but I had no problem playing the 5 games that Mike mentioned would not load. I noticed one of them took a minute to start since it froze on a black screen until the system quit credit feeding after pressing start with freeplay on. This is the same exact board so it has to either be a coin-up issue or something else within the options.
One thing about rapid fire: it can be turned on/off at any time in any game by hitting start + A, which I found extremely handy.