Disadvantage of using plastic?
All plastics are petroleum based products. i.e. they come from oil.
Oil is a finite resource and many other problems are associated with oil extraction and usage.
During the production of plastic vast quantities of toxic waste are produced by treating them with flame retardants, plasticizers and various other organic compounds.
Plastics have a very high residence time within the environment most items of plastic are used for 20-30 minuets before they are discarded sometimes lasting 1000 years within the terrestrial environment. This usually ends up within marine sources. The Pacific Ocean has one of the largest dumping ground for plastics, unknown numbers of sea birds marine mammals and fish ingest plastics which causes a variety of negative health effects.
As plastics break down by wave action, photic or biological means generation of microplastics also occurs. Microplastics have the potential to harbour persitant organic pollutants and polyaromatic hydrocarbons which are highly toxic to all known living creatures.
However plastics only beome a problem when disposed of incorectly by dumping or buring. Plastics have made the 20 and 21st centry much easier for the human race to thrive and live better lives. There are no disadvantages to using plastic as for its many purposes, just disadvantages in the way humans dispose of plastics.