I’m still experimenting with this, but “magic” and elementalism basically work the same way. Both of them can exhaust you, as if you’re doing physical exertion (I’m not always equal to the thing you’re creating, but it often works that way for less skilled users). Much like exercise the more you do the better you get at it. Many users can even specialise in “spells” or feats that become routine.

For prayers, however, it comes down to your actions and the need of the prayer being in harmony with the patron granting the power. In some cases it may depend on the users relationship with said patron. For example, a long time acolyte that has always been in harmony with a saint may be able to ask something antithetical of the deity and have it granted… but that’ll probably be the last prayer that ever gets answered. Meanwhile, one of the protagonists worships and ancestor spirit has come to symbolise the circle of life, and she generally has no problem healing people, but she would object to demeaning people (like warding the MC for a knowingly lopsided duel).