There are a number of disadvantages to shared hosting but they will not affect everybody that uses the service. For those with personal websites, these disadvantages will have no effect. However for those who want to start up a business, they may run into a few problems.
The main problem is the number of websites that are on the server. This can cause servers to run slow, which will make the websites run slow. This is very problematic for those who have a lot going on within the website or need a strong connection for shopping carts or other high server-side programming.
Another problem is that the IP address for all of these websites is the same. When someone has been blocked by a country for their content, the IP address will be blocked rather than the domain name. This means that everybody on that IP address will be blocked. This is very bad for those who are attempting to run a business that will reach the whole world.