Bioenergy is one of the renewable energy sources with high expectations in terms of its potential for greenhouse gas (GHG) emissions mitigation, and thus has been included in most global warming limiting strategies and pathways. However, within this context, a state-of-the-art and comprehensive understanding of the environmental performance of currently available bioenergy technologies is still missing. Hence, we conduct this critical review on life cycle assessment (LCA) studies regarding a wide portfolio of bioenergy technologies to deal with this lack of knowledge. Our critical review of exhaustively searched literature identified commonly existing limitations and difficulties in the selected LCAs in terms of essential aspects of LCA, i.e., system boundaries, functional unit (FU), multifunctionality, and impact categories. Key findings of our review are as follows: inconsistency of system boundary definitions, incomparability of LCA results due to various FU definitions, incomprehensiveness of impact categories, as well as a lack of uncertainty and sensitivity analysis. Finally, in view of the above findings, we present a generic guideline for future studies with the purpose of overcoming the identified shortcomings.