  The gauge bosons of Pati-Salam do not mediate proton decay at the
renormalisable level, and for this reason it is possible to construct scenarios
in which $SU(4) \otimes SU(2)_{R}$ is broken at relatively low scales. In this
Letter we show that such low-scale models generate dimension-5 operators that
can give rise to nucleon decays at unacceptably large rates, even if the
operators are suppressed by the Planck scale. We find an interesting
complementarity between the nucleon-decay limits and the usual meson-decay
constraints. Furthermore, we argue that these operators are generically present
when the model is embedded into $SO(10)$, lowering the suppression scale. Under
reasonable assumptions, the lower limit on the breaking scale can be
constrained to be as high as $\mathcal{O}(10^{8})$ GeV.
