Anaerobic digestion (AD) and composting are commonly used methods for the biological stabilization of sewage sludges (SSLs). Both solid digestate and compost produced during these processes can be used as organic fertilizers, provided standards are met. However, the presence of potentially toxic elements (PTEs) that remain in the solid digestate and compost poses a risk of their release following soil application. This paper examines the transformations (persistence and bioavailability) of PTEs (Cu, Zn, Pb, Cd, Ni, Cr) in both stabilization processes of SSLs with particular attention to Cu and Ni, which exhibit distinct behavior under different stabilization conditions. Research has shown that AD generally leads to an increase in PTEs content in SSLs, whereas composting may result in either a decrease or an increase in PTEs content in the final product. In composting, reduction is associated with the leaching of PTEs to the compost leachate, determined by the pH level of the composted matter. In most cases, AD and composting are environmentally beneficial, promoting transformations of bioavailable exchangeable and carbonate-bound or reducible fractions of PTEs to stable oxidizable and residual fractions. The introduction of passivation additives during both processes positively influences PTEs immobilization. Available studies demonstrate that AD and composting effectively stabilize SSLs and significantly reduce PTE mobility and bioavailability, thereby lowering potential environmental risks. Appropriate optimization of process conditions allows for control over PTE transformations, enhancing the safety of solid digestate and compost as organic fertilizers.