Originally Posted by
WizardOfBoz
Maybe for tubes, but for tubeless, it's pretty easy to gunk up a valve core.
Also, I've seen (in my cores) where the rubber gasket in the core that seals things actually rolls out of it the groove its supposed to be in. Pretty sure this had to do with sealant, but if not, there's a justification: you can fix the valve by replacing the core without removing the tube and tire.
Incompatibility of the sealant...specifically the glycol in the sealant...and the o-ring. The o-ring swells and won't fit in anymore. See it all the time in valves on reactors when the wrong o-ring is used.