Unlike Coinbase and other services , Blockchain.info don't control our private keys and don't have our password stored in their servers but It's hackable , anyone with access to their servers could try to crack the wallet.dat and get some $$ in his hands with no effort as some passwords are weak.
I have one question could someone can also get hands in the bitcoins that was imported in blockchain.info if they crack wallet.dat file? And even getting access to wallet.dat file from blockchain.info server hacker will have hard time to crack individual wallet which they even don't know how much bitcoin it carry.
However it is true that no any web wallets should be considered as safe place to store bitcoins.
If by importing the wallet.dat (private keys) means swapping them to other private keys on blockchain.info servers then the imported wallet will be useless but If someone puts his hands on the wallet.dat on the servers on blockchain then cracking it would be possible. wallet.dat contains a lot of information's[1][2][3] and there is some tools to read them[4] so I assume It's possible to read the addresses + balance without the need of cracking it, after having the addresses that contains a lot of
, the cracking begins process begins.