  We consider the generation of neutrino masses via a singly-charged scalar
singlet. Under general assumptions we identify two distinct structures for the
neutrino mass matrix. This yields a constraint for the antisymmetric Yukawa
coupling of the singly-charged scalar singlet to two left-handed lepton
doublets, irrespective of how the breaking of lepton-number conservation is
achieved. The constraint disfavours large hierarchies among the Yukawa
couplings. We study the implications for the phenomenology of lepton-flavour
universality, measurements of the $W$-boson mass, flavour violation in the
charged-lepton sector and decays of the singly-charged scalar singlet. We also
discuss the parameter space that can address the Cabibbo Angle Anomaly.
