  Waveguides in nonlinear materials are a key component for photon pair sources
and offer promising solutions to interface quantum memories through frequency
conversion. To bring these technologies closer to every-day life, it is still
necessary to guarantee a reliable and efficient fabrication of these devices.
Therefore, a thorough understanding of the technological limitations of
nonlinear waveguiding devices is paramount. In this paper, we study the link
between fabrication errors of waveguides in nonlinear crystals and the final
performance of such devices. In particular, we first derive a mathematical
expression to qualitatively assess the technological limitations of any
nonlinear waveguide. We apply this tool to study the impact of fabrication
imperfections on the phasematching properties of different quantum processes
realized in titanium-diffused lithium niobate waveguides. Finally, we analyse
the effect of waveguide imperfections on quantum state generation and
manipulation for few selected cases. We find that the main source of
phasematching degradation is the correlated variation of the waveguide's
dispersion properties and suggest different possible strategies to reduce the
impact of fabrication imperfections.
