While I appreciate the sense of what you say, my experience is that the ACA is actually more fragile (by which I mean, faster to go down) than, say, the Phoenix Adept or the Justicar, both of which seem more fragile on paper. The ACA gets one-hit downed more often when I play it than the others I mention.
I suspect that you may be onto something with the movement speed thing. I tried playing the ACA today using Adrenaline Mods and I seemed to spend less time going down.
I have also tried a build without Seeker Swarms and it seems to me that he/she/it is less fragile without the SS DR. Which makes no sense, unless the SS DR is bugged out, which I'm not ruling out at this time.
Peddro and co tested the reckoning characters quite through, I'm sure they'd have found that out, so I'm pretty sure it's not bugged

What Capn said actually makes perfect sense. Each race has hidden attributes regarding their speed in general and, more importantly, their acceleration, which one inevitably fails to account for when thinking about how survivable a kit is/should be.
And yet, given how much damage the enemies cause on gold+, and how enemy accuracy works in this game, when it comes to make a kit more survivable the amount of shields and health gets actually trampled by the ability to reach soft cover faster and in a more reliable way.
Which explains, for example, how can Drell be very survivable kits when played the right way, much more than, say, a Krogan Soldier, which in theory has a helluva lot more shields/health. And yet, with the enemies being able to down them in fractions of a second, and with their accuracy reaching 100% after you spend too much time in LoS... Those shields won't help much

This. I cannot keep this guy alive.
Move. Relocate more. That's all there is to it... Relocate to the next soft cover place, and if you find yourself going down a lot it usually means you need to start relocating sooner.