extremely difficult! at least by my standards! Basically the only method of getting the data out of them is by allowing them to boot in a GBA and then loading a dumping program into memory while keeping the power on, which is tricky enough, but then you have the issue of "trap addresses" where it will lock back up if you try to read certain addresses & you just have to trial-and-error your way to figuring out where they are. There's more to it than that, I've written up the method here https://github.com/tzlion/vfdump/blob/master/YJDumping.md

However after dumping the ROM this way it only needs minimal modification to work, it doesn't need to be cracked further nor need special emulation, the protection on these carts is 100% read protection really