I'm obviously super biased, but I think the core installation process is nearly as straightforward as it could possibly be without official support. All of the cores are distributed in a way that you can simply unzip the asset to the root of your SD card, and the core is set up and ready to go.
The only part I would really have to complain about is the folder names, particularly for ROMs. For SNES for example, ROMs are stored at `/Assets/snes/common/rom.smc`, where obviously `common` is an annoying addition to the path.
Huh what? That's absolutely not true. I just went to https://github.com/spiritualized1997/openFPGA-GB-GBC and followed the instructions: All you have to do is copying files onto a microSD card. Custom tools are not needed. You can skip firmware checks (wcgw).
All my Gameboy ROMs play without any issues so far. Yes, you have to pay attention to put the files in the correct directories, but I wouldn't call that a "disaster".