The biggest disadvantage with building a computer is the lack of any one support organization you will be dealing with. Since each component can and likely will come from a different manufacturer and/or store means that if a part has a problem, you will have to deal with the appropriate company. With pre-built systems, you only have to deal with the manufacturer and their warranty service groups.
The second biggest disadvantage with building a computer system is cost. In most cases, the cost of building a computer will be slightly more expensive than purchasing a pre-built computer. This is due to the fact that the manufacturers can buy in bulk for steep discounts that translate into savings on the computer system. They also include software with the systems that must be purchased separately when building a system. OEM software purchased with hardware does help reduce the costs on custom built systems but it is not only OEM software that companies include in their systems. The systems are always riddled with software that a good number of people do not need and only serve to take up hard drive space. Some of the added “Bloatware”, namely, games and related are trial software. This software often leads people into downloading more software then was intended by offering other software during the install process. Many do not pay attention to this due to the way it is presented. The extra software is presented in a way that would appear like a simple software agreement. Just when you think the agreement is just for the software you are installing it actually is an agreement to install another program, game, or browser search bar.
The cost disadvantage does not always hold true though. As the more money one is willing to spend on a computer system, the more likely you will be able to save money by building the PC yourself. This has to do with the top of the line specialized systems are often marked up for extra profits per unit by the manufacturers. So, if you are looking to spend big, investigate building it yourself first. As I mentioned latter, you never know what you are getting on the inside due to availability of great pricing for the better parts. When having a PC built for you, you will know what is inside and if your builder is good and honest he/she will choose quality parts which most of the time translates into longer lasting parts.