The only way I can see you getting actual value out of the widowmaker compared to the shotgun is if you need to fire more than six shots in a given combat instance.

But engineer is inherently not a good class for long sustained fights, so getting the most out of this weapon requires a medic to pocket you. The problem with that is that there's better classes you could be pocketing as a medic, like a pyro, soldier, demo, or a heavy, heck even a scout might be better if you are willing to put that much cooperation into it.

Point is, the widowmaker is at best a meme weapon that does some good shooting sounds but for most cases you are probably better running with the stock shotgun as the widowmaker tends to also make you overconfident which is a death sentence for an engineer.

If I had to propose a change to fix my gripes with the widowmaker, I would give it a faster firing speed so it can feel like you're really quickly pumping damage into the enemy and being rewarded for being accurate.