The urgency of the energy transition, combined with geopolitical tensions over critical raw materials, has increased interest in deep-sea mining of polymetallic nodules. However, there are serious concerns about its environmental impacts. Life Cycle Assessment (LCA) is currently used to support claims that metals from deep-sea mining are environmentally more preferable than those from land-based mining. However, LCA addresses only generic environmental impacts, and previous LCAs comparing deep-sea and land-based mining show considerable variation in outcomes. This study critically examined the applicability of LCA for comparing deep-sea and land-based mining by analysing previous LCAs, developing an aligned LCA, and discussing limitations. Previous LCAs consistently reported lower climate change impacts for deep-sea mining, but the magnitude of this benefit varied due to methodological choices, data inputs, and assumptions, especially regarding energy. The aligned LCA found lower impacts for deep-sea mining in almost all 20 evaluated categories, except transformation of natural land, including the seabed. However, the results depended strongly on LCA design choices: under renewable electricity scenarios, land-based mining outperformed deep-sea mining for climate change and energy resources. Trade-offs emerged, such as high water use from hydropower and ionising radiation from nuclear energy. The study highlighted both strengths and limitations of LCA in this context, and emphasized that it should be complemented by location-specific assessments to capture social, economic, biodiversity, and ecosystem impacts. This study did not conclude whether deep-sea mining is environmentally more preferable than land-based mining, but it identified conditions under which it results in lower generic environmental impacts, and highlighted critical considerations, knowledge gaps, and uncertainties to guide policy making and future research.