Ammonia inhibition remains a major constraint in sludge anaerobic digestion (AD), particularly in THP systems where protein solubilisation can increase total ammonia nitrogen (TAN) accumulation and free ammonia (FA) exposure. This risk was reflected in previous batch biochemical methane potential (BMP) tests using conventional AD (C-AD) sludge, THP sludge and food waste under increasing FA exposure from 44 to 1200 mg/L. C-AD sludge was strongly inhibited at 250 mg/L, while THP sludge showed delayed inhibition, with marked volatile fatty acids (VFA) accumulation becoming more evident around 700 mg/L. Osmoprotectant also improved methane response under intermediate FA stress. Most full-scale sludge digesters operate continuously, hence, batch results cannot fully predict continuous digester stability, performance and biomass washout effects. This study examines whether ammonia inhibition is better defined by fixed thresholds or by system-specific microbial resilience. Four mesophilic CSTR configurations were compared: C-AD, with and without osmoprotectant, THP-AD, with and without osmoprotectant. Reactors were exposed to step-wise increases in FA from 50 to 160 mg/L for C-AD, and from 120 to 500 mg/L for THP-AD. Methane response, VFA accumulation, and TAN/FA exposure were used to identify stability boundaries and support ammonia risk management.