I've been having trouble with 4700 Tiagra rear derailleur for winter use. It seems to get gunked up fairly easily, at which point shifts become a bit erratic (I have new gear cable that has shifted well even after the stretching period so it's not that). I'm now trying cleaning it, dousing it in WD40, waiting overnight then and keep lubricating it with chain oil every night for a few days.
I should probably watch the GCN video on refurbishing a rear derailleur. I posted this in advance so everyone can laugh if I've fucked it up.
But I do have to say that the mountain derailleurs I have (XT, SLX) are far more tolerant of dirt.