I played Ash in yesterday’s sortie as a bit of a lark; I thought I would have to be carried, based on my time with Ash in the Plains.
Quite the opposite, as it turns out—Blade Storm, doing finisher damage, and slash procs, doing finisher damage, are a potent combo against high-level Grineer armor. I was at the top of the damage and kills charts, in particular on the final-stage defense mission with level 100 enemies. I ended up using Blade Storm almost exclusively. Not only is it some guaranteed invincibility to let shields recharge, it also vacuums up things dropped by your targets, like energy orbs.
On that note, the most important part of an Ash build, in my opinion, is 75% efficiency (and probably Zenurik and/or Energy Siphon). All of Ash’s damage and survivability comes from abilities, and they don’t need a tremendous amount of power to be endgame useful. (I think I’m running the standard Continuity/Intensify/Streamline, plus Fleeting Expertise to top out efficiency, but I don’t remember offhand.)
I still say Ash gets a bad rap as far as meta goes. Between Ash, Inaros, Trinity Prime, and Frost Prime, I feel like I have a solid stable of endgame frames. I’m still a little short on weapons, but I’m building Opticor right now, which I’ve been seeing a lot of lately.