What are the disadvantages of PBX?

Can you please elaborate on the potential drawbacks of utilizing a Private Branch Exchange (PBX) system? Are there any notable limitations in terms of scalability, cost-effectiveness, or maintenance requirements? Additionally, how might the reliability of PBX systems be impacted in the event of power outages or natural disasters? Lastly, are there any concerns regarding the potential security vulnerabilities associated with PBX systems, and how can these risks be mitigated?