Polymer-derived ceramic nanocomposites (PDC-NCs) can be synthesized via thermal conversion of suitable single-source precursors, leading in a first step to amorphous single-phase ceramics, which subsequently undergo phase separation processes to furnish bi- or multiphase ceramic nanocomposites. PDC-NCs have been shown to be excellent candidate materials suitable for applications at ultrahigh-temperatures and under harsh environments. In the present work, amorphous SiHfCN- and SiHfBCN-based materials were synthesized via cross-linking and ceramization of tailor-made single-source precursors. High-temperature annealing of the obtained amorphous ceramics led to UHTC-NCs with promising compositions, such as SiC/HfC, HfN/Si3N4/SiBCN or HfC/HfB2/SiC. The presented results emphasize a convenient preparative approach to nano-structured ultrahigh-temperature stable materials starting from greatly compliant single-source precursors. Additionally, recent results concerning the stability of the prepared UHTC-NCs in ultraharsh environments (i.e., oxidative atmosphere, combustion atmosphere, hydrothermal environment) will be presented and discussed.