We investigate the cosmological implications of Tsallis entropy in two widely discussed frameworks: the Cai-Kim thermodynamic derivation of the Friedman equations and the Tsallis holographic dark energy (HDE) scenario, considering both the Hubble scale and the Granda-Oliveros cutoff as infrared regulators. In both cases, the dynamics introduce a nonextensivity parameter $\delta$, with the standard Bekenstein-Hawking entropy-area relation recovered for $\delta =1$. While previous studies have suggested that only small deviations from extensivity are observationally allowed, typically requiring $|1-\delta |\lesssim {10}^{-3}$, here we go further and perform a systematic consistency analysis across the entire expansion history. We show that even mild departures from $\delta =1$ lead to pathological behavior in the effective dark energy sector: its density can become negative or complex, its equation of state may diverge, or it can contribute an unacceptably large early-time fraction that spoils radiation domination and violates big bang nucleosynthesis and CMB constraints. Our results sharpen and unify earlier hints of tension, providing a clear physical interpretation in terms of corrections that grow uncontrollably with the expansion rate toward the past. We conclude that within both the Cai-Kim and HDE formulations, a viable cosmology emerges only in the extensive limit, effectively reducing the models to $\mathrm{\Lambda }\mathrm{CDM}$. More broadly, our findings emphasize the importance of dynamical consistency and cosmological viability tests, when assessing nonextensive entropy formalisms as potential frameworks for describing the Universe’s dynamics.

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• Alternative gravity theories