As Coinbase generate a new bitcoin address for every deposit and further use this funds for other users transaction, I think it will be nearly impossible to reveal cold wallet or major exchange wallet address.
Besides, this is a matter of security to hide such address. While other present themselves as a transparent exchange, showing all the address, Coinbase has chosen a more secure tactics.
Yes and we should remember that is in indeed *difficult* but not *impossible*.
For a very smart person who knows what he is doing he can for sure find the address, but rest it's also dependent on the person who is using the wallet also since at the end of the day personal errors are the ones which causes most of the problems.
What am more concerned about is that , Coinbase would ofcourse have all the data stored and they are more likely to undergo an attack , people have been trying a lot hard to make sure that the site is compromised but I have not seen some major news regarding them. There are few instances where people's personal wallet was hacked but then again it was the faltu of the people and not Coinbase itself.
I do think that cold storage wallet address are way more secure and the only one who can cause a problem would be the Coinbase's storage data itself.