In recent decades, fusion energy for electricity has become an international issue with worldwide interest in several magnetic fusion concepts offering the most promising energy source for this century. From existing experiments to power plants, several next-step facilities (NSFs) must be built to bridge the large gaps in fusion science and nuclear technology. During the course of fusion studies, all power plants and NSFs require an integral nuclear assessment to identify the nuclear parameters and address key issues related to tritium breeding ratio (TBR), blanket design, selection of low-activation materials, radial/vertical build optimization and definition, magnet protection, shielding, activation, and survivability of structural materials in 14-MeV neutron environment. This paper presents our design philosophy, nuclear assessment approach, and recent research results for ARIES conceptual tokamak, spherical tokamak, and stellarator power plants as well as NSFs. Some features of the nuclear activities [such as tritium breeding requirement (overall TBR = 1.05), blanket concept, and radwaste issues] remained fixed between the various designs, while others [such as service lifetime (20 to 200 displacements per atom) and shielding requirements] were subject to change to meet the specific design needs. Emerging challenges and lessons learned from nuclear assessments performed during recent decades are highlighted throughout the paper. In particular, the cost implication of uncertainties in the TBR prediction and the large amount of low-level waste generation are important challenges facing the fusion community and should be addressed by interdisciplinary research programs.