Shotguns are not worth it right now, because the game balance is built that way.
1. It is firing extremely slow. For example, lev.1 char kills an imp in 2 shots with both a chaingun and a shotgun, on average. However, you might pull off firing twice in a row with a chaingun, whereas after you fire a shotgun once - imp may get enough time to throw his fireball twice. And after THAT you still have to spend another turn reloading.
2. Shotguns get their own dedicated perk, Shottyman. However, it means that if you take it - two levels of Reloader are essentially wasted. Not only that, but you are still pretty vulnerable while reloading, so you must also get Hellrunner, and STILL firing pretty slow, so you must also get Finesse...
Look at pistols now. You take two levels of Son of a Gun - you get access to Dualgunner, but Son of a Gun levels keep working for you ! Well, they are broken now, but they should, in theory, anyway.
3. If that was not enough - further in the game shotgun ammo gradually stops appearing. Chaingun ammo is still plentiful.
I love shotguns, but in recent versions i am rather reluctant to even try winning a game solely with them.
Track earlier versions, shotguns were stronger then.