The biggest limitation to PayPal Payments Standard is that transactions are not processed in real time.
When the customer goes through the checkout process with Paypal standard and get to the Checkout success, this only means that paypal has accepted the payment information. the transaction has been placed in a queue and will be processed on a first come first served basis. The transaction may be processed in as little as 5 minutes, or as long as 5 days.
Once the transaction has been processed with the bank, or credit card company, payPal will either email you or you can set it up to do an IPN post to let you know if the transaction was truly successful, or if it failed.
because of this, it becomes very difficult to deliver digital goods using PayPal standard. people expect to be able to download digital goods immediately, but you have now way of knowing the status of the transaction immediately when using PayPal standard.