You could think of the sites that share your server as your roommates; there's really not that much separating you from them. Sure, you can close the bedroom door, but they can still cause nightmares for you in the kitchen and the bathroom. In web hosting terms, all the sites share a single server's resources, so huge traffic spike on Site A may impact the neighboring sites' performances. It's even possible that another site could take down the shared server altogether, if it crashed hard enough.
One thing we learned in reviewing the services listed here (and many more) is that even though the packages are very similar, they are not identical. Some are more security-focused than others, offering anti-spam and anti-malware tools. Others offer a variety of email marketing tools. While most of the hosts we've reviewed have built-in e-commerce, you may want to consider using a more-robust third-party online shopping cart application like Shopify instead.