I don't think this is what Just to Browse meant with their post, but I feel like a lot of the "nightmare" would come from initially crafting the system in the first place and getting people familiar with it. (and justifying your new spell levels to the players.)
Because you've got an insane amount of spells to sift through and apply a level for, even before breaking into sourcebooks. Unless you develop a system that determines which spells land in which levels, but then you have to run each spell through that system each level to determine if a character is ready to cast it or not.
Not that this should discourage any of you homebrewers out there to attempt such a thing...