1. Incompatible Systems
Let's say you start out with a small system, then want to expand with another system to cover the entire house. Oftentimes, the 2 systems will not work well together. Research compatibility before you buy or buy from the same brand.
2. Landline Dependence
Believe it or not, some systems still rely on a land phone line in order to send out alerts. Obviously, this means a thief can simply cut your line to render the system worthless.
3. Inadequate Coverage
Especially for larger houses, it is difficult to cover every angle, every corner of the house/yard. Default systems may not have enough sensors. Choose a system that allows for easy addition of sensors to the coverage area.