if SB goes bust your shares/cash in your trading account are safe (but the process could take years before you get them back).
This site is pretty much hack proof being in 128 bit cryptology and the risk is you use an easy to guess password, loose (give away) your password or your PC gets hacked with something like a keyboard sniffer. If somebody does get into your account all they can do is buy/sell or transfer money back to the account that the money came from (your account). If they tried to add a new account to pay money into FICA would trip them up.
So in sort we have two risks - both us. We loose our password or buy/sell the wrong stocks.