Often interest is charged when paying with instalments. This means the company/organisation you are paying charges you extra for being able to pay it off over time. So in the end, you end up paying more.
Another disadvantage is that some people buy a whole load of things on instalments and they over commit themselves and cannot afford to make the payments when they are due.
An advantage of buying something on instalments is that you can buy something that you don't have all the money for immediately because you are paying it off over time.