Because in shared web hosting server resources are shared between a number of accounts, if traffic to a particular website spikes, it will utilize more of the resources available than the other websites. This means that the performance of your website is often at the mercy of other, better performing web sites on the same server. In addition, being shared, hacker activity, malware, viruses, and any disruptive activity (DOS attacks, etc.) directed at a particular web site could impact all the accounts on a server.
Furthermore, because of users’ lack of control of server resources, they are wholly dependent on the support provided by a host. If support is under par, this can lead to tremendous frustration on behalf of the user. The quality of support is key to most people’s decision regarding which web host to use.