Sintering is a key microstructural evolution process in powder-based additive manufacturing, such as binder jetting, as well as in feedstock-based processes including material extrusion, where interfacial-driven densification governs the final geometry and material properties. To enable predictive simulations in such manufacturing contexts, microstructure models must be not only physically sound but also numerically robust and reproducible. Phase-field methods have therefore become indispensable tools for modeling sintering and related interface-controlled processes. However, when applied to sintering, they pose pronounced analytical and numerical challenges arising from strong surface-energy-driven dynamics and evolving time scales. In this contribution, we present an energy-consistent phase-field modeling framework for sintering processes, focusing on viscous sintering as a benchmark problem. Within a unified variational formulation, surface-energy-driven interfacial motion and densification are consistently described. Particular attention is paid to the numerical stiffness that emerges during early-stage sintering, where surface tension dominates the dynamics, followed by a gradual relaxation as the system approaches equilibrium. This intrinsic time-scale separation renders conventional fixed time-stepping strategies inefficient or overly restrictive. To address this challenge, an adaptive time-integration strategy driven by the instantaneous free-energy dissipation rate is introduced, ensuring numerical efficiency while preserving discrete thermodynamic consistency. Representative sintering scenarios are used to analyze stiffness regimes and to identify quantities suitable for benchmark-oriented comparisons across different phase-field implementations. Finally, the transferability of the proposed energetic and numerical framework to diffusion-controlled solid-state sintering is briefly discussed, highlighting methodological commonalities across phase-field models for interface-driven microstructural evolution.