I personally do the first in spring. it lets you see the 'framework' of the Sambucus.
I prune my escallonia after it has flowered so the new growth has chance to harden before winter. I have removed growth in spring but found i got fewer flowers that season.

Escallonia are normally very tough plants so its up to you when you would prefer to do it.

I certainly wouldn't hard prune Escallonia now - they are reasonably hardy, but certainly quite a few were completely defoliated and even died in the last two winters.
As for the yellow leaved Sambucus, you can do it when its dormant, in winter, so next month really, or the end of this month. When I had one of those, I used to hack it back early to mid August because it looked so tatty I couldn't stand it, and it did fine.

Glad you said that, Bamboo! I 'hacked back' two Sambucus.....in late August! Couldn't stand them any longer either!