The key problem is that they cause deforestation:

Bio-energy with carbon capture and storage (BECCS) has been touted as a clean way of producing baseload power, substituting for gas and coal, which could even result in “negative emissions” as when replacement forests are grown they take up CO2 from the air.

But such systems could take 150 years to be “carbon negative”, researchers from the US, UK and China have found, in part because of the long time it takes to regrow forests, and because of the damage done when existing savannah, pasture or cropland is converted to grow biomass for burning.

The paper is here