Municipal biowaste is a major environmental issue. Life-cycle assessment is a valuable tool to assess recycling options, and anaerobic digestion and composting have performed adequately. However, reviews indicate several discrepancies between studies. Thus, we critically review 25 life-cycle assessments of the composting and anaerobic digestion of municipal biowaste. Our objective is to identify decisive factors, methodological gaps and processes that affect environmental performance. We generally identified methodological gaps in expanding systems borders. In energy systems, the replaced energy source did not consider power generation or dynamic regulation. All studies adopted mixed energy sources or marginal approaches. Agroecosystems included the carbon sequestration potential and compensation for the production of synthetic fertilizers only. A limited range of scientifically proven benefits of compost use has been reported. In general, studies provided a limited account of the effects of use on land emissions, but contradictory assumptions emerged, mainly in modelling synthetic fertilizer compensation. Only three studies compensated direct emissions from the use of synthetic fertilizers, and none included indirect emissions. Further studies should include an analysis of the additional benefits of compost use, compensate for the effects of emissions from synthetic fertilizer use on land and mix attributional and consequential approaches in energy system expansion.