the first of these is the fact the customer is sent to paypal every time they click on a add to cart button, this is ok if only buying one item but if you wanted more then one it can and will get annoying and result in potential customers going else were. second keeping track of sales, stock, customers details, thirdly updating is time consuming.
there are scripts out there that can be incorporated in to Bluevoda that will help with displaying cart items two demos can be seen
here and
here which are basic but show the customer all the details they need to know before going to checkout, and i believe that Navaldesign has a all in cart that will incorporate into bluevoda.
Good luck with what ever you decide.