Vulnerability plays a big role for sure but let me expand, through numbers, the problem that COs using this plane set typically must face.
Where ships (as here) are the target, then the problem of minimum ord to crack the target becomes a major issue of force composition.
Let's look at the Carrier/Cruiser combo and the scoring in this FSO simply by way of example. The designer guides the CIC's planning by providing incentive through scoring. The higher the score for the target, the more certain you can be that the designer wants you to hit it.
This FSO the designer has allocated 144 points for a carrier and 72 points for a cruiser. It will take 8000lb and 2000lb of ordnance in direct hits to score the points. Even a lb less and you get nothing.
To solve your allocations, you have to look at the load carrying capacity of the strike aircraft in your arsenal and determine minimum carriage capacity to determine the number of aircraft and types to use.
Let's look at the Allied set first.
A B25 can carry 3000lb of ordnance. A single trio carries sufficient ordance to sink a carrier. A single aircraft carries sufficient to sink a cruiser.
The TBM can carry 2000lb. Four planes required for a carrier and one for a cruiser.
An SBD loads up 1000lb needing eight planes for a carrier and on 2 for a cruiser.
Against that minimum load for effect you have factor in a multiplyer. This figure takes account of the skill set of the pilots and the likely vulnerability of your aircraft to the defenders. It his higher for moving targets than fixed targets.
If I was sending B25s against a carrier I might choose a 3 to 1 ratio (YMMV but I am setting out a problem faced by all CICS). This means that I would send 2 trios of B25s against a carrier/cruiser combo and then mulitply that by 3 to get sufficient bombs out to have a certainty of effect for effort. That is a total of 6 trios per shipping group minimum. That's 18 planes but only 6 pilots. (I get 12 extra people in fighters now to protect assets). 180 points risked.
For TBMs I'd use the same ratios and come up with 4 + 1 required on target with a total of 15 airframes (and pilots) for effect. 75 points in the ring.
SBD's are different. Their more accurate delivery provides some relief on the multiple.
You need 10 singles for target destruction but can happily drop to a 2 to 1 ratio given their accuracy and toughness. 20 SBDs for a pair of ships and you just about get a guarantee of 216 points for an exposure of 80 points risk.
Now for the Axis arithmetic.
Big old Betty carts 2200lb thus 6600lb in trios. You require two trios to hit the carrier and one to get the cruiser.
If you were able to use the same ratio as the other side (you can't because of vulnerability) you would require 9 trios minimum for an effective strike at the target. This is 27 planes at 11 points per plane (yes there is a greater penalty for losing a G4M than a B25 in this one) which is a risk of 297 points for a possible gain of 216. You get to use less pilots as escort because you have to have more in bombers.
You are required to use (and risk) 24 minimum at 11 points a loss.
Having less planes to escort means your bombers are now more vulnerable. But wait! They are already more vulnerable because AH determined that they should be destroyed easily. Now you have a deep problem. The only way you can start to guarantee that the target gives you anything back at all is to further increase that multiplyer up to 3.5 or 4. Remember you get nothing at all if the ship stays floating - it's all-in each hand.
Looking at the other types:
Vals carry 770lb requiring 11 hits on a cv and 3 on a cruiser. 14 planes perfectly striking required in all but a mulitplyer of 2 might suffice in the right hands hands. 28 planes at a cost of 4 gives 112 risk against 216 gain. Thats 8 more than the SBDs meaning that you lose a squad that might have been able to escort them.
B5Ns can carry 1760lbs which means 5 for a carrier and 2 for a cruiser. A simple 7 X 3 = 21. You might choose to use a lesser multiple given the ability to dive bomb, but I have used 3 because of the lack of effective self defence. Risk 105 for a possible reward of 216.
The A6M3 doesn't carry ord but the Ki43 carts a useful 1100lb. Thats 8 for the carrier and 2 for the cruiser. Multiple is probably just a bit above 2 as it is not a dedicated dive bomber but in the hands of fair pilots you can get a result. Take 25 and you risk 75 points with a fairly good chance of getting a few home.
So what does that tell us? Well it possibly means that you should expect to see lots and lots of Ki43s in the air for the final frame.
This possibility however doesn't at all help answer my original question:
How is it possible for a design to incorporate that plane (the G4M) but not be so unbalancing?
There must be genuinely useful ideas out there.
My genuine inquiry is about how we can encourage its use in events and little to do with the mathematics showing that it has less than slight appeal to CICs and pilots.
The old "G4M model 11 24 MIN" will ensure their use, but I'm sure there are more attractive ways to incentivise the use of an airframe we begged and begged for.
