I can only say that it works just fine. I know someone who has a character with all three of those powersets and does very well. (Sorry, I don't have the build. I can tell you it's based around L/S defense, has around 50% recharge, and includes purples and PvPOs.) The AoE output is quite fair between DT, Fireball and Burn.
Edit: Sorry, had originally posted his latest time soloing Pylons, but that's a newer KM/FA.
Blue
American Steele: 50 BS/Inv
Nightfall: 50 DDD
Sable Slayer: 50 DM/Rgn
Fortune's Shadow: 50 Dark/Psi
WinterStrike: 47 Ice/Dev
Quantum Well: 43 Inv/EM
Twilit Destiny: 43 MA/DA
Red
Shadowslip: 50 DDC
Final Rest: 50 MA/Rgn
Abyssal Frost: 50 Ice/Dark
Golden Ember: 50 SM/FA
My latest project is MA/Fire. Only level 26 right so far. I'm finding the sets endurance intensive and lagging just a bit on survivability then what I usually run in this level range, but fun both on teams and solo. I feel kind of like a blaster, in that I can't jump into spawns yet, but if some else draws agro, I can run in and play clean-up.
My plan for the endgame is a high recharge build for extreme ST DPS while having lower then benchmark solo scrapper survivability (a hole easily filled by an insperation or a teamate).
I think you're correct that this isn't a very common combination, although since both sets were buffed about a year ago, they are more common then they used to be.
I gotta make pain. I gotta make things right. I gotta stop what's comin'. 'Least I gotta try.
Seems like an uncommon pair.
I think about rolling one, with the Blaze mastery for AoE and concepts.
Any build or advice about it?