A very young, wild or inexperienced dog with sheep could cause a problem. . . . . but so could not being able to move them. The way I look at it, is that if the dog is relatively controlled and working well, moving them with a dog is probably the least stressful way to do it.
As said, it just takes the ball ache out of routine jobs. I.e yesterday I had to wean lambs, so three good sized mobs of ewes and lambs, to pen up in the field (no handling area) and draft off ewes, ewe lambs and ram lambs, load each group into separate trailers (or decks of trailers) and then drop them off in designated areas (i.e bring all ewe lambs together, all ram lambs and penning all ewes on poor grazing to dry off). The mobs were on some big and quite rough areas. . . . . without the dog doing one mob would be most of a day gone, making clever hurdle funnels, and chasing the odd awkward ewe. With the dog is just worked. . . . . and the amount of joy I get from working her is second to none! Same as you, my old man is getting on, and its just me, looking after around 220 sheep (mine and some I contract shepherd). . . . there is no way I could do it without the dog!
Also during lambing its a god send!