There are lots of different ways to punch: There's perfectly vertical like kung-fu / wing-chun does it, also known as a "half-twisting" punch. There's the 3/4 twisting punch like your krav-maga and Okinawan karate tends to do. There's perfectly horizontal like Japanese karate and Taekwondo does it, also known as the "full twisting" punch. There's the corkscrew punch after that, which you're going to see most often in boxing.
So which is right? Which is better?
The answer is: It depends, all, none.
There are advantages and disadvantages to all of them. While the risk of breaking your finger bones or spraining your wrist is higher when punching hard objects like heads, no particular punch does better in this regard. That's provided you're doing them right. And to know if you're doing a punch wrong, you'll want to punch a heavy bag to see whether or not it would cause any problems in real life.
That's absolutely vital. If you don't train using a heavy bag or some kind of hard resistance, it won't matter if your form is technically perfect, you're just going to end up hurt. You need to condition your body to punch solid objects. And you can condition your body to punch in any of these ways without getting hurt.
So, let's just get that out of the way. None of these punches will end up hurting you any more or less than any of the others, so long as you're hitting a heavy bag and conditioning your body to punch with those punches.
If you do actually find that you get hurt when punching the bag even if you're going light, then it says that the punch you're doing can't be done without hurting you. So you learn not to do that punch. It's just that simple. This takes just a minute or two to figure out.
What you'll notice with a half-twist punch (the vertical fist that kung-fu / wing-chun likes) is that it requires you keep your elbows in and pointed down. The force you generate is with your legs rather than just punching with the arm. There's very little biceps or triceps involved in the punch. And it feels like you're driving a wedge down the center line of the body. People that use it will just keep walking forward and use the force of stepping itself to attack. Since you're not swinging it from the outside, it lends itself well to short range fighting. Martial arts which use center line theory utilize this punch as their main punch.
But that half-twisting punch has some problems. First, it often relies on short power, which really depends on how skilled you are at it. Beginners will have very weak punches. It's debatable, but even experts at it tend to have weaker punches than western style boxing punches would be. But power is not really the goal with half-twisting punches. Or I'll say it's not the only goal. The main focus is to deliver quick, somewhat powerful strikes down the center line in order to attack your opponent's structure. And for that, it's fine.
The worst part about the half-twisting punch is that it leaves you wide open for punches to your face. Your elbow and shoulder are down. You can't deflect a hook punch to your head without taking the time to move first.
A western style boxer has this figured out by utilizing the extra, downward twisting turn at the end of the strike. Ever notice that about boxing punches? Ever wonder why they do it? It seems "wrong" to most people.
Ramsey Dewey did a whole video about this, which you should see:
His take is that you do that corkscrew turn at the end of the punch in order to raise both your elbow and your shoulder. That provides a shield to defend against punches to your face while you're performing the punch.
It has nothing to do with power, but rather, with defense. Or at least, it's debatable whether you can get any extra power or lose power by doing that extra little twist at the end.
So, power isn't the only goal of a good punch. It's just one of many goals. If your punch leaves you wide open to punches to your head, then your punch is kind of "weak" in a sense, regardless of how powerful it is.
Let's get back to the 3/4-turn punch that you mentioned from krav-maga, and which is similar in some ways to that of Okinawan karate. Now that you understand how important the shoulder and elbow are in defense, do you see now how the 3/4-turn punch might leave you more vulnerable than the horizontal punch or the corkscrew punch would?
In terms of biomechanics, the 3/4-turn punch does cause the bones of your forearm and your first two knuckles to align. Again, it's not super important that those bones align in terms of avoiding damage to your wrists or the bones of your hand. Punching a heavy bag will condition you to punch in any way without hurting yourself. And if the argument is that you can avoid hurting yourself even if you're not conditioned by utilizing this 3/4-turn punch, the answer is no. You will get hurt no matter what, so long as you're not conditioning yourself to punch.
Okinawan karate also adds a dipping motion to the first two knuckles that krav-maga does not. But you can only do that motion when you're doing a karate style lunge punch down the center of your opponent through his torso, rather than punching to the face. It's a very penetrating punch.
Now here's what I think. It's my judgment that boxing has the best understanding and application of punching of pretty much any martial art or self-defense system. They take a very dynamic and realistic approach to it. They're not just looking at power. They're looking at the entire game. I think one year in a boxing gym, going 2 to 3 times a week would give you a better understanding of punching than krav-maga would give you in decades of training. Or at the very least, you would find it complements your krav-maga style and gives you a base you can use for performing other krav-maga skills. My opinion, anyway.
What I hope you walk away with after reading this is that there's a lot more to punching than just biomechanical structure. Above all else, put it to the test. See how it does in the ring and on a heavy bag. And speaking of that heavy bag, you need to be hitting it hard in order to condition your body to punch for real, or else it won't matter how technically perfect your punch is.
Hope that helps.