I mean, that's just Sturgeon's Law. There are great and terrible examples of books with both defined and undefined magic systems, no argument there. Unless you think that ratio is unique to this one fantasy subgenre?
In any good book, the audience should have a clear picture of who's the strongest, the smartest, the most charismatic. Whether or not your spells are prepackaged with distinct power levels and number of uses doesn't really affect that. An author can demonstrate every bit of info the audience needs to follow the action, simply by the way characters react and interact with each other in each scene.Not due to any particular Deus Ex machina moment but just because I have no idea what anybody can actually do at any given time. It usually manifests in the opposite, actually, where a character suddenly CAN'T do something because the plot would be ruined if they could...but there's not real reason for WHY they can't do i when they were able to do all this other stuff earlier. What do you MEAN you can't heal a gunshot wound, "Doctor" Strange?
If you need more explicit details than that to feel immersed in a story, no judgment from me, but that's edging into "personal preference" territory.
This is true for the reverse too, though: you need to be very good at writing to pull off a defined magic system, because otherwise you'll use it as a crutch. People like learning the rules and limitations of a fantasy system, so if your writing is shaky you can usually pad your story with long explanations of how your magic system works, and that will build investment in your audience because it's easy worldbuilding. Throw in a few cool interactions you thought of and you can resolve the plot entirely on the strength of the magic system and create a feeling among readers that you gave a satisfying resolution because all the pieces technically fit together...even though you didn't really do any serious work with the characters or themes or writing.I like a good undefined magic system but it requires the author to overall be MUCH better at writing to pull off a satisfying story.