Biomass refers to the energy that is derived from organic matter of plants and animals. It belongs to renewable energy sources but when burned it is not totally clean source of energy.

There are still doubts whether biomass is really carbon neutral or not. Carbon neutral means that biomass fuels only release the same amount of carbon into the atmosphere as was absorbed by plants in the course of their life cycle.

Biomass power plants are quite expensive.

Biomass power plants have significant land requirements.

Increased biomass production, if not sustainable, can lead to less food and more fuel, and thus lead to more hunger in the world.

Fossil fuels have history and powerful lobbies behind them which makes is hard for biomass sector to fully thrive.

Fossil fuels still have edge in efficiency over biomass. For instance gasoline is more efficient than ethanol fuel.

Since wood is one of the most widely used biomass sources, increased biomass production can lead to deforestation and thus have negative environmental impact.

Transport and resource gathering expenses connected with biomass production are high. 

Clearing plants and organic material from the earth can also lead to adverse health effects of surrounding soil that requires biomass for compost and fertilization.