I think that bombers *are* balanced in the game.
In scenarios, bombers work such that, (1) when they are unescorted and jumped by a bunch of fighters, most of the bombers are going to be shot down, and (2) when they are strongly enough escorted, most of the bombers get through. That is appropriately realistic. Also, in both cases, the bomber pilots with better gunning skill tend to have significant advantages in survival, and when bombers get to target, the bomber pilots with more skill in bombing things destroy more objects and achieve more objectives. So bomber-pilot skill matters a lot. Also, on the opposite side, fighter-pilot skill matters a lot. There are guys who are terrors of the sky, and average pilots who aren't nearly as dangerous to bombers. In most cases, unless the bombers never see an enemy, whether bombers are all shot down or not, there is plenty of action in the process for the bomber pilots and the fighter pilots.
All of this works well enough that bomber pilots and fighter pilots tend to have lots of fun in bomber-centric scenarios like Der Grosse Schlag and feel that things are well balanced.
In the Main Arena, they also seem balanced. If you go in on the deck and get swarmed by attacking fighters, you will get shot down. If you go in with altitude and have to fend off an attacker or two who isn't so skilled and if you are a decent gunner, you can make it to target. If a super-skilled fighter pilot attacks you, and you have no escort, you are in big trouble. If you have escort, you can often make it to target.
All of that seems reasonable. Bombers are neither so weak that they never get to target nor so powerful that they always get to target. That is balance.