http://invest7xci3j4tfs37zeyxylwmhfnl2nvfbf6geos4xxeh3ejmzntfid.onion/bitcoin-calc/index15.html
For one thing, transactions are often slow. For another, any transaction has to pass through an intermediary, like a bank, meaning there is a central point of failure. And there are issues in keeping track of all accounts and balances; data can get jeopardized, manipulated, or even corrupted across multiple systems where the accounts and balances are maintained.