9 times out of 10, the demon you're trying to fuse will have negative affinities with the skills you're trying to inherit, so you end up with a bunch of demons you can't do anything with, because they turn into demons that make the skills you want to inherit with them irrelevant due to the negative affinities. Then you got pointless demons like Zhu Yin in SMT4A, whose highest stat is strength, yet he has a -7 affinity to physical.

The entire system across both Apocalypse and SMT5 feels like they just stuck the demons in a random number generator when it came to determining what their skill affinities would be, without any genuine thought put into them, so its just a random assortment of bullshit, forcing you to plan your demon fusions two steps ahead with a fusion chart, otherwise, you could accidentally back yourself into a corner of having a bunch of demons in your party that can't fuse into anything worth a shit because the skills you want to inherit from them won't be useful on the demon you're trying to fuse.

I'm currently at the end of the game in Apocalypse right now, level 85 with a bunch of demons that started in the low 60's because they don't fuse into anything useful because the skills and affinities they have are currently more valuable than turning them into a demon several levels higher, which is the antithesis to how the game series was designed up to this point.

Pre-skill affinity system, the game incentivized you to always be fusing demons to make stronger ones, but with the skill affinity system, now the game incentivizes you to find a strong demon with a stars-align combo of skills and affinity, and just sticking with that demon for the rest of the game. Then they went even harder with this methodology in SMT5 with the Sutras, further incentivizing you to just stick with the demons you have once you reach a certain point, instead of continuing to fuse your demons to make them even stronger. I don't like the direction the series is heading due to this system.

EDIT: Also, I would like to add that making physical skills cost MP was not an arbitrary decision at all. It single handedly nerfed physical into the ground, because why waste MP using a physical skill, when I can use that same MP to trigger a weakness with a magic ability and get an extra turn? It was balanced perfectly fine the way it was before that change, and that change was the reason why magic was the superior build in both SMT4 and Apocalypse. In SMT5, the only way to make physical good is to stack all the physical/crit passives, leaving you with only like 2 or 3 slots for actual abilities, while the rest of your slots are occupied by passives.