I've had 4-aco-mipt and 4-aco-dipt that definitely got darker and more granular, as opposed to the pure white powder they came as, that showed no appreciable difference in potency. I've read of highly degraded 4-ho-det that was black and still fully active (for clarification, it was 4-ho-det I read about, not necessarily the same batch as referred to above). As has been said already, highly recommend storing 4-subbed dialkyl tryptamines in an amber class vial (often available at headshops) at around -15 degrees Celsius for the long term.
I've never had an issue with a 5-meo tryptamine or simple n,n dialkyl tryptamines showing any noticeable degradation as long as properly stored. I imagine if someone forgot some in a simple baggie in a glove compartment or something on a hot, humid day, rapid degradation could occur.
Edit: I would be remiss to mention that none of my tryptamines were the fumarate salts so common these days, they were HCl salts obtained around the time of WebTryp.