Depends. If the discontinued devices are from a major maker, such as Mutable Instruments, Intellijel, etc, there's not likely to be any worries, plus these may in fact be easy and cheap finds on the used market (in some cases). But when it comes to smaller boutique makers, it's likely best to stick to ones whose output has been ongoing over the long term in case technical issues arise. Also, if the device in question was a kit build, be very cautious unless the build was done by a builder with extensive experience. My Digisound 80, for example, was a kit build...but it had been restored and cased/powered by the legendary Kevin Lightner (RIP), one of the greatest synth techs ever. So even though it was a kit build, Kevin had gone over all of the functions and corrected any faults that might have been present because...well, that's what he did. He was a picky guy in that way.
Again, as with anything used, it's a caveat emptor kinda joint. When first getting a "new" used module, check all of the functions, controls for noise, dropouts, jumpy control behavior (all of which tend to indicate dirty controls, etc), and make sure the patchpoints are snug and electrically solid. If you have an oscilloscope (definitely a tool serious modularistas ought to have on hand!), examine LFO and VCO waveforms for spectral purity and proper waveshapes. Also, VCFs can be checked similarly by bringing them into self-resonance and examining the output for a good, clean sine wave. Mixers and VCAs can be checked by sending known pure waveforms through them and looking for distortion components, and waveshaper functions can be checked against their settings by observing how they manipulate a simple waveform. Dirty pots and switches can usually be sorted out with some of Caig Labs' wonder-drug, DeOxit, and badly-performing jacks can either be spritzed with that same magic formula, or you can obtain a jack burnisher to scrub crud off of contacts.