Described herein are thermoset compositions suitable for hybrid polymerization when processed via additive fabrication equipment utilizing sources of actinic radiation with peak spectral intensities in the UV and/or visible region containing a photoinitiating package, a cationically curable constituent, a free-radically curable component, and optionally, one or more additives. Such thermoset compositions preferably contain a Norrish Type I photoinitiator that is an alkyl-, aryl-, or acyl-substituted compound centered around a Group 14 atom, and further possesses specified ranges of ionization potential values with respect to its known triplet state. Also disclosed are methods of creating three-dimensional parts via additive fabrication processes utilizing sources of actinic radiation with peak spectral intensities in the UV and/or visible regions employing the claimed thermosetting compositions, along with the parts cured therefrom.