Some companies will place up to 1000 accounts on a server - this means you share that computer with 1000 other sites, and your sites performance will suffer. Pages will be sent slower and you'll lose customers.
Some companies oversubscribe their bandwidth.
That simply means they sell more bandwidth than they have, and hope you don't use it all. The hosts costs are less, so your cost is less, but so is your performance. Finally, knowledgeable tech support is key, even if a host has 10 people, if they don't know what they are doing nothing will get done. Netfronts has a staff with 2 Master's degree and everyone has at least a bachelor's degree. We know how many sites to place on a server so that it never gets overloaded - especially at your critical peak usage hours.