Updated Post (2021/05/12):
It's safe to say that this implementation while seems impressive at first glance, it is extremely impractical for real use with any ROM hacks, and I advise anyone to avoid this type of implementation as it has provided nothing but trouble with the hacks I've worked on fixing. I admire the effort, but I have to say this is quite a hacky solution for something I don't feel is worth jumping into given the current nature of the implementation. The guide itself wasn't even followed completely by MeroMero as part of the edits weren't even finished in the patches they provided (part of me believes they knew that as well), and their guide itself when followed correctly yields a ROM image that doesn't work, I've went in at least four times now repeating the steps carefully and every time I've had nothing but blank boot screens. I've even went as far as using other tools in substitution of CrystalTile2 thinking maybe that would yield different results, it did not unfortunately. Nothing personal against MeroMero either by all means, but this should have not been released in this state, it should have been tested much more thoroughly.
Original Post:
This is a very impressive implementation of the fairy type mechanics, however it has some flaws that break compatibility with real hardware. I took a look at this documentation and looked at examples of ROM hacks using this implementation for the fairy types. Pokemon Moon Black 2 beta 3.0 came out, sporting so many new features and of course, the fairy type implementation. Now I am not well acquainted with Pokemon Moon Black 2, however I have been around the game since it's first release (I update the cheat databases with new hack versions when I can so it's sort of mandatory for me to know). Pokemon Moon Black 2 beta 2.2 was able to work on any hardware you threw the game at, whether it be flashcarts on the Nintendo DS, TWiLightMenu, DraStic on Android, DeSmuMe, etc. however that clearly isn't the case with the new update, which only seems to work only on methods involving emulation and not real hardware. I decided to do a closer inspection of why this was happening and stumbled upon a very nasty sight. I found out that arm9 was exceptionally larger than usual, also some of the assets of the game were also larger as well (though these assets don't seem to be the problem or at least part of the cause), while one could assume it's because the ROM hacker (JrFort) added some custom content to whatever he messed with, it actually turned out to be the content that was tampered with related to the fairy type mechanics implementation. I decided to go to next level and test the integrity of the fairy type implementation myself with a vanilla copy of Pokemon Black Version 2, I made two copies, one for the patch provided in the first post and one for me to manually edit with the set of instructions given above. Upon inspection of the patched version I noticed that arm9 in the vanilla fairy type patched version was also larger than the usual size, so I went in and took a look at the steps to apply such changes to the game manually. It didn't take too long to figure out why arm9 was so big, apparently the result was due to the extraction process done via CrystalTile2, while the software isn't necessarily at fault, the process in which the data is implemented is the reason why real hardware compatibility is broken, it's also the reason why the boot time of the hack takes a bit on emulation (even though it still works on emulation). There have been plenty of other known cases were certain hacks have had problems like this, Pokemon Silver Yellow is another example, while yes that hack is a generation four ROM hack, it too suffers the same issues with the way fairy type mechanics were implemented. Pokemon Renegade Platinum has fairy type implementation as well, HOWEVER, implementation in that ROM hack was actually done correctly, none of the assets nor arm9 are bigger than usual, thus resulting on full 100% compatibility on real hardware and or emulation. Therefore I'd like to request that the fairy type mechanic be implemented differently so that it's not breaking compatibility on anything, thank you.
Here is an image of the results of fairy type implementation on a hack and how it breaks compatibility. As a side note, the header was part of the issue, I fixed that though which made the game able to boot, but it hangs still because of arm9.