What are disadvantages of network computers

They're connected to other computers -- Meaning your files can be accessed, malware installed, etc. Basically (if another computer is in the hands of a skilled individual) everything you do on your computer can be seen/accessed/modified by the other person, including bank account/credit card info and the like. Also, network cards generate a slight amount of heat, can reduce airflow inside of your computer case, and add a bit of weight.