The main reason not to support PayPal standard and suggest paypal express instead is that paypal standard is not a real time processor.
When the user gets to the thank you page, the transaction has been placed in a queue for paypal to process with the merchant bank, that can take any where from a few seconds to a few hours to actually happen..
paypal express though, processes the transaction in real time, if the user is directed to the payment success page, that means the funds have been transferred from the merchant bank to your paypal account.