I would imagine the main issue with covers is Hololive being too big. The people to whom the original belongs think about how much revenue the girls could generate (disregarding that they generally go red and a lot on making them), abuse the fact that all it takes is for them, at any point in time, just say "No" and it all falls through, and ask for more money after the work on it has already begun, effectively holding them hostages. Either they cut the losses and cancel a cover, or pay a lot more than expected to owner. Enjoy your fotm covers from Kanaria from now on as the only possible options, because Kanaria seems to be chill with it. Music copyright is probably the most anal one about "muh lost revenue", to the point bands, who wrote the song, sometimes can't perform it if they are no longer under a label they were back when it was done