Many companies use subcontractors to install the system.

You, the "purchaser" do not own the system.  It is a hidden lease.

The equipment is usually the cheapest cost and quality available.

If the equipment fails, you remain locked into the contract for full term.

If you move, you remain locked into the contract for full term.

The "system" consists of only a few alarmed doors or windows.

All previous employees of the installing company know the installers code.

Central station costs are almost double normal costs.