OP weapons and class limits in TF2

What are th weapons that need tweaks in TF2? Why? How to fix them? Why do weapon bans and class limits exist?

SCOUT:
- Soda Popper
- Bonk!
- Crit-a-cola
- Mad milk
- Sandman
- Atomiser

SOLDIER:
- Disciplinary Action
- B.A.S.E. Jumper

PYRO:
- Reserve Shooter
- Homewrecker (Maybe not OP, but in my opinion, if you manage to get a kill on an engie while the pyro is around, he deserves to get his buildings sapped, without having a free and easy "Out of Jail" card.)

HEAVY:
- Natascha
- Tomislav
- Sandvich
- Gloves of Running Urgently
- Fists of Steel

ENGIE:
- Rescue Ranger
- Pomson 6000
- Wrangler
- Short Circuit

MEDIC:
- Crusader's Crossbow
- Quick-Fix
- Solemn Vow
- Vita-saw

SNIPER:
- Sniper Rifle (sort of, more like with the concept of Sniper as a whole)
- Sydney sleeper
- Machina
- Darwin's Danger Shield
- Jarate
- Razorback

SPY:
- Diamondback