@nan0bitz Yeah, I'm not denying the benefit from her point of view. Although personally I feel like she should broaden her spectrum of directly used magic from healing to other stuff, most importantly self-defense. Still, it's one thing to watch practice matches and then heal whatever scrapes and wounds might appear, but a totally different situation to witness a real battle.
I talked about negative effects on physical growth, but I 100% expect it to not get addressed by the series at all, so it's kind of a moot point from me. Exercise enhancing a person's fitness is linked to getting physically stressed and exhausted by training, which signals the endocrine system to secrete growth hormones and other relevant agents, enhancing muscle growth. Or in simpler terms, if the body notices more strength is needed, the body will grow more strength. However, if Sei removes the exhaustion, it will also remove the need for the body to get stronger. It would absolutely be a good thing in a real battle, but not in exercises.
What I was also referring to is developing a mindset of it being okay to get wonded because the wound will be perfectly healed rightaway. Or it being okay to take big risks because failure doesn't mean anything with the instant heal available. It's exactly the same as a character/monster learning to rely on OP regeneration ability and then getting miserably defeated when that regeneration is turned off for whatever reason.