Any author worth their salt will implement consequences or some form of penalty for characters who use magic. You cannot walk, or run, or swim without expending energy, and likewise you cannot think, or perform any sort of mental activity without doing the same. Nevertheless, some characters may be more adept at using magic and have greater endurance than others, and this can be reflected in racial traits, and/or experience and talent. I’ve seen floating continents or islands in many fantasy worlds, but few of them ever explain what actually keeps them aloft. If anyone ever says “well it’s magic”, I will shoot them with as sarcastic a comment as I can think of. What, pray tell, is controlling that magic? Is there a natural disturbance in the atmosphere? If so, what caused it? Did some ancient magician use a powerful spell to lift it into the air? If so, how has it remained aloft for centuries or millennia if said magician is long dead?
No fantasy world is believable if it has a magic system that the author cannot explain. It might be asking too much of an author to explain every single detail, but they should be able to tell you, during the course of their stories, enough so that it at least makes logical sense. Physics may work differently or be more malleable in a world that is not Earth, so if you have more than one moon in the sky, the author should be prepared to explain that one as well if the ocean tides are no different to that of our own world. There must always pros and cons, for without them there is little to no clarity. Even the most supremely powerful being needs some form of weakness to make them believable, even though to the rest of the world it might seem as though they have none. There ought to be some penalties somewhere along the line.
This has just been a short piece to explain my stance on how I believe the use of magic in fantasy worlds needs to be explainable in order for readers to make sense of it. Speculation can only be taken so far after all. I hope you find it useful.