So this weekend I finally finished the campaign and I got a lot of quest gear which was great because I did it on one of my season 1 alts who didn’t have a high ilvl.
This morning I was like okay cool I can skip it now on my alts because… I hate questing. Lol. Even though I love Undermine, I’m not keen on doing that questline again just for catchup gear.
However I wonder if doing it again might be more or less efficient than doing delves / world quests.
If I were you, I think I’d base my decision off of how many fire irons you may have left over from the Siren Isle, if you have any. You could toss those to your alt and use that as a catch-up, if a bit lower ilvl compared to the quest rewards. Alternatively, see if you can outfit yourself in ilvl 597 greens on the AH. The price fluctuations on them look WILD when I scroll through them, but if you can get each piece for 100g, that sounds like a good enough deal to justify the story skip (as vendoring them later will net you at least half of it back).
Weapons look especially cheap to “rent” this way, due to their high vendor price. Buy for 90, sell for 85 later, and your stopgap effectively only cost 5g.
All my alts did the campaign quests in Undermine because they just give a LOT of valorstones which you can run out of while upgrading gear early in the patch.
I ran out of crests long before I ran out of valorstones. 
Hmm well, I completely skipped siren isle (looked dumb to me lol), so that isnt an option. Thanks though
I’m not even sure how to skip the undermine campaign.
Lol so after a while of figuring this out, I’m actually not done with the campaign. I have to defeat Gallywix in the raid. Which… I’m not sure how im gonna do that as I’m not going to go in there on normal and the LRF wing wont be open for several weeks.
Aaaaan after even more digging, found out you cant do it on story mode until next tuesday