The reason providers can offer their “Unlimited Plan” for a cheap price is because they are overselling. The companies know that most customers are not going to use very much space and bandwidth. For those that do, the company is either large enough to afford it, or it is not. You should be on the lookout for those in the latter category. If the company cant afford for you to use all the space and bandwidth you are given, you can count on them having some hidden terms that prevent it.
You may find some sort of clause in their Terms of Service agreement that states that you may only use a certain percentage of your space or bandwidth, or something like that. They wont allow you to get anywhere near the advertised limits before your account is suspended for ‘abusing’ the limits given to you.
You may also notice poor service from hosting providers that are extreme oversellers. Websites and download will run very slowly, and you may experience frequent downtime.
The most important thing to remember is that you should always carefully read the Terms of Service before signing up for any hosting account. Sign up with a reputable company that offers reasonable limits and you will be much better off than with a disreputable one offering high limits.