Spatial environmental heterogeneity is widely assumed to enhance ecological stability by promoting refugia, biodiversity, and asynchrony. Yet, we lack field experiments testing this fundamental relationship and its underlying mechanisms in naturally-assembled multitrophic systems. To address this gap, we monitored experimental substrates replicating topographic heterogeneity on a rocky shore over three years. Contrary to theory, heterogeneity showed no net effect on community stability due to four counteracting mechanisms. Heterogeneity increased stability by i) providing refugia that enhanced population stability and ii) boosting species richness, which promoted asynchrony. At the same time, it decreased stability by iii) suppressing consumers and iv) reducing a dominant non-native species, both of which otherwise stabilised community composition. These opposing processes cancelled out the heterogeneity-stability relationship, highlighting the complex and multi-causal nature of this relationship. We caution against the assumption that increasing heterogeneity universally enhances stability, particularly in systems with strong consumer interactions and dominant species.