Assuming you mostly pug, and are just doing it for the keys, then I would say just the older dlc worth it, since there is little difference in terms of difficulty (WGT, ICP, CoS, and a few others).
For Dragonbones DLC forward, HM can be a bit challenging, and the time spent for just another key doesn't compensate. On the other hand, if you are on premade group or you got a very good pug, some HM can be interesting as they have guaranteed motif drop, but thats it, only for eventual motifs and/or nice titles, never for pledges per se.
So it sounds like post for most DLC dungeons the pledge becomes a SLIGHT bonus for completing them then. I mostly run pledges for transmutation stones, or try to.
It seem most people think that the scale of the rewards for vet HM DLC pledges do not mach the difficulty. Hopefully a future update will include an increased amount of stones and keys.
I've only managed to clear a few veteran dlc hard mode pledges in all the years I've played ESO. And in my opinion the rewards are not worth. Can I get a few second opinions?