Nb–Si-based ultrahigh-temperature alloys are considered promising candidates to replace conventional Ni-based superalloys in hot-end aerospace components operating at 1200–1400 °C owing to their high melting point, low density, high-temperature strength, and creep resistance. Their microstructures generally comprise a niobium solid solution (Nbss) and silicide phases, with the Nbss providing room-temperature toughness and the silicides imparting high-temperature strength and thermal stability. However, the inherent brittleness of the silicides, insufficient continuity of the Nbss phase, and the difficulty of forming a stable protective oxide scale at elevated temperatures create a long-standing trade-off between room-temperature fracture toughness and high-temperature oxidation resistance, thereby severely limiting the engineering applications of these alloys. This review focuses on the relationships among processing technology, microstructure, and properties in Nb–Si-based alloys. The effects of directional solidification, arc melting, powder metallurgy, and additive manufacturing on phase constitution, microstructural characteristics, and properties are systematically summarized. Particular attention is given to the roles of Ti, Zr, Mo, Cr, Al, B, Ta, Hf, and rare-earth elements in phase stabilization, microstructure optimization, strengthening, toughening, and oxidation resistance. In addition, the synergistic and antagonistic effects of multi-element alloying are discussed in terms of suppressing Nb₃Si formation, stabilizing beneficial silicides, maintaining Nbss continuity, and promoting the formation of protective oxide scales. Finally, current challenges, including the lack of unified performance evaluation criteria, insufficient understanding of composition–process coupling, and limited design strategies for simultaneously enhancing toughness and oxidation resistance, are highlighted. Future directions for the design and development of high-performance Nb–Si based ultrahigh-temperature alloys for extreme service environments are also proposed.