They are actually really difficult. There's no instrumental mixes available to work from and the instrumentation is itself difficult for vocal removers. Of course .. the AI technology is moving forward at incredible speeds, so it will get easier. Karaoke Nerds website is a great resource for the time being as a lot of Karaoke creators gather there.
I'd love songs from their Collide album like Fingernails but I understand how difficult it can be. I'm just happy to see Skillet sings at all and I thank you, CC.