While I agree with your statement regarding likely recoil spring life in the shorter action guns, I don't agree with your generalization regarding the G42 and reliability.I think it’s the recoil spring. 3” barrel semis have low tolerance for spring rate deviations, whether recoil or magazine. The lock timing and cartridge rise have a smaller window than longer barrel guns.
A lot of 3” guns have low spring life numbers.
This is why Glock basically stayed at the 3.4” length with the 43. Basically the same lock timing as the 26 which is duty gun reliable.
The 42 can be a tad shorter b/c if the shorter 380 cartridge, but it had still shown to be not quite duty gun reliable.
I would like sig to get it together on the 365. The action and FCU group would likely be very reliable in a larger type pistol.
I own a G42 that has been flawless since new. For certain, over 50% of the 5K+ rounds I have shot through it are running at 1200fps with a 90gr projectile. I still have the original recoil spring in it, managing substantially more free recoil with 100% confidence.
I think any meaningful reliability issues have been addressed with factory upgrades.