From Gamesverse Wiki
The issue of magical lethality and the canonical restriction on mass weaponry is one that serious thought and no small amount of argument went into. This project has settled the issue thusly:
Simply put, modern magic is generally less-than-lethal, and much safer than kinetics. This is for several reasons. The first is the almost universal usage of Barrier Jackets, which are generally optimised towards defending against magic rather than kinetic weaponry. As well as this, if you observe things like the Divine Shooter bullets, they're both slower and less concentrated than a bullet, meaning a lighter impact spread over a greater area. They're beanbag rounds, designed for non-lethal incapacitation. The TSAB have laws in place mandating that all legal combat spells must have safeguards in place, built into the spell, which require conscious effort to override to the point where they will kill a target. For instance; a shooting spell that delivers an sledgehammer-like impact of variable force to something is required to have safeguards that you need to override in order to make that transmitted force large enough to kill someone. If it does not have these safeguards, it is illegal to use the spell, in the same way as it is to carry an unlicensed firearm on Earth.
That said, even with the safeguards, the bigger spells are still dangerous. If we look at Starlight Breaker, we observe that even when Fate was hit by it, her Barrier Jacket survived, albeit highly damaged. It's probable that if you didn't have a Barrier up to soak it, something like Starlight Breaker would just vaporise you. That's another reason for Nanoha to not use it, quite apart from its other restrictions; it's a wide area spell that can kill. Things like the Divine Shooter can be micro-handled by the Device for their target; hit less hard against unarmoured targets, and the like. With Divine Buster you still have a single target, but you're only using it against a single foe who's probably armoured and there's still a risk of collateral damage. Starlight Breaker is not a safe weapon; it relies on your foes being armoured, and most magic attacks will be smaller than that, with safeguards built into the spells when they're designed so that they take deliberate effort to make lethal when used on other magic users. Time-Space Distortion Barriers are used to evacuate civilians who aren't likely to have armour, and the TSAB try hard to keep fights away from population centres.
All that said, this applies to modern magic. Earlier forms of magic such as those developed in the Dawn States were not less-than-lethal as a default. If Shamal blasts you, she will be shooting to kill. Vita's little metal balls don't care if you're unarmoured. Signum is actually setting you on fire. Those pillars that Zaphira used to pierce the drones would have done the same to an enemy force. The Wolkenritter will have to learn to put in safety features in their spells, which is one of the differences of Neo-Belkan style from Old Belkan Style.
In short, modern magic is less-than-lethal as a default due to the systems that the TSAB use being set up that way, and measures being taken to enforce it. While it is possible to make a Midchildan or Neo-Belkan spell without safeguards - which applies to any other modern system just as much - it is illegal to do so without authorisation. It is still possible to kill with magic, especially if you're using it on civilians without a Barrier Jacket, but when fighting other mages, you're using blunted weapons, padded blows. It is important to note that this does not render them harmless. Blunted weapons and padded blows can still break bones, burn skin and even damage internal organs. It does, however, remove a lot of the risk of death, rather than injury. With kinetic weapons like guns, there is little that can be done to to make them less deadly but still effective, and for this reason they are prohibited.