That's a good point. I think for light novels (where the plot more often than not isn't entirely planned out in advance), there is a sweet spot somewhere where the author has some freedom, but it still has enough of a structure to not feel arbitrary. Slime Tensei did something in that sense where there is a framework of rules at play, but with a Deus Ex Machina in the form of Raphael, the author can just break free if needed. Sadly it feels a bit inconsistent in later volumes.

Personally, I greatly prefer the planned-out, rigid magic systems as long as the author has the skill to pull it off. Even if the constraints aren't so well defined, it's always a huge hype moment when a character can overcome those limitations (ie. Bookworm).