It's about assessing the current risk models of the clients (different FI institutions) and providing the recommendations. MBB companies rarely create their own risk models since these projects are too long and the clients cannot afford hiring MBB for them.
In terms of the learning - it's mainly the financial knowledge. Also based on what I've seen at McKinsey it requires a lot of travel since the practice is not that big.
Exit opportunities - obviously the banks' risk management departments. Or any other company / role where you can sell your McKinsey experience without mentioning that you were working primarily on risk.