Fistly, I'd like to know why direct combat spells were chosen for this nerf. From my experience, most mages use guns in combat as there's normally a chance of rolling fewer hits than you hoped and damaging yourself. Why do you introduce mechanics like this only for direct combat spells and not other spell categories like mind control?

Since dice pools became static (give or take foci choice), mages as they progressed have faced the choice between increasing casting pool and increasing drain pool. Now for every 3 dice you add to drain pool, you can on-average increase the drain value by 1, meaning you can cast it at 2 higher force without fear of taking drain. That's 3 more dice in your drain pool to do +2 damage, but with this system you're needing 3 more dice to deal one more damage. Shouldn't it at-least be +1 drain for every 2 hits used to increase drain, to keep it even?

The rules say nothing about allowing a mage to hold-back hits, so as to do less damage yet face lower drain. Let's say that I have a pool of 10 dice to cast with (that's less than most runners) and roll 6 hits (better than a 1/20 chance). My opponent has an attribute of 3 and scores 1 hit. Now I'm doing +4 damage (possibly unnecessary) yet I'm facing +4 drain (which, combined with the base drain, is probably going to put me at-least on wound modifiers). On the flip side, +4 hits is a critical hit, which gives you a point of edge back, but then you start needing to use edge because you rolled well.

Let's say you're attacked by a group of spirits on the astral plane, and throw a stun ball at them. Most of the spirits take a little damage, but the presence of a watcher amongst them means that one of them scored 0 hits to resist and so you're at +3 drain from your casting. The weaker an opponent is, the more dangerous it is to throw a spell at them. In-fact, I could see a mage opting to give a weak opponent spell defense just so they could hit a large group without fears of taking too much damage.

Even against an individual weak opponent, I could see a mage deliberately giving themselves pool penalties so they didn't do too much damage and cripple themselves. They could cast at a low force and see how many hits they got, but then they're only getting 1 damage per drain rather than the 2 damage they'd get by casting at a higher force.

All of the above scream of a badly designed rule. Good rules encourage the players to play how you want them to. If you consider players squinting to give themselves vision penalties, giving opponents spell defense so they don't cripple themselves, and spending edge to avoid some particularly good luck to be how you want the game to be played, then this rule might be good. I don't, however, and I really don't like the thought that some table somewhere might be using this most poorly thought-out rule.