It's a shame so few people are playing Sub, although with the nerf to the legendary boots, Master of Shadows + Subtlety I can see why more are playing Assa. Assa is even getting a flat +4% dmg buff to abilities soon.
Even so, I really enjoy the spec, and I started off as Outlaw, and Assa, after going through all of WoD as a Sub rogue.
Luckily DfA doesn't get you killed on Krosus (
I haven't tested when he smashes the bridge though- there I sprint at4-6 seconds before the smash and get to trigger Flickering Shadows ), but it can one Elisande. It's one fight it's not too great on, due to mostly 1-2 adds on her, and that you can get killed by the Nightwell itself is she ports when you jump up.
A gif of it happening to me last night.
https://gfycat.com/EasygoingGiddyKingsnake
When it come to Alacrity, the only two bosses I think it's worth using on is Skorp, and Botanist. Since you focus entirely on a main target, so your stacks don't really fall off that easily.
Otherwise you stick with Premed for the extra combo points.
Also Eviscerate Finality is consumed with DfA, so getting to 9-10 CP, Evis'ing and then popping DfA is good for extra damage.