Demogirl (N7 Demolisher) recommended builds

You can go full cheese by maxing nades (radius and anti shield on the arc ones, split grenades on the homing ones), power damage and being fitneless. Launch fist the arcs, then homings for tech bursts. Choose extra grenade on Pylon, and equip a light weapon (maybe pistol with power mod). This way you can "spam" Pylon, and each time you deploy it you'll instantly have 2 nades. And, by deploying the Pylon, the old one explodes causing damage to enemies. This way you can kill someone with your pylon and get the achievement "incompetent engineer".

Demo Girl. This is my gold build. My build for plat is a Cyclonic instead of a Power Amp. Other guns you could use would be Carrier, Black Widow, Talon, Venom, Claymore, etc. 

I think split homing nades are better than armor damage single one.

Arc + Split nade = Lotsa dead mooks. For armored enemies just split nade and Crusader to face. Carrier is obviously a good choice too, since you have your own ammo box.

My preference is to entirely spec out of Homing Grenades in favour of getting a full-health build - and in that way I can equip a Power Amp instead of a Cyclonic. For weapons, I like the Scorpion (cos it staggers,  it sets up tech/fire explosions efficiently and Phantoms hate it) with Pistol Power Magnifier and Heavy Barrel. To chomp through heavy armour, I carry a Hurricane (with Heat Sink and High-Velocity Barrel) as a secondary. For the higher difficulties, I like to equip Incendiary ammo (who doesn't? ) and a Pistol Rail Amp. It goes without saying (but I'll say it anyway ) that Grenade Capacity is a must for a 'nade-spammer.

I use a rather classical no-fitness build with power amplifier module (cyclonic for solos). As Vastator explained, her thing is to chain tech bursts with Arc grenades + Homing grenades. As for the weapons, there are plenty of good choices, but with her I like to have one which stagger (she's very fragile without fitness and cyclonic) and one with a good dps (or I find her too weak against bosses).

Please elaborate...

well if u got two homing nade with small radius, each of them will detonate a TB (wide priming by arc nade, 7.8m), as long as the second nade detonation is not in the first nade detonation radius (lol 2m).

Ahh, the classic demogirl. I actually played her for a while at one point when I wasn't using my VMS.

Not sure which if I ran radius or damage on both grenades first, but I think I ended up switching to damage later on, but radius is really good too. And don't remember why I picked this weapon, but I used the executioner pistol back then, which wasn't half bad on her, but I switched to the arc pistol later on.

Ran no fitness, full power damage on passives, supply pylon with with resupply, power damage, extra grenade.

In terms of how I played her, for mooks it's self explanatory, arc grenade and split home nades and get tech bursts as others have mentioned, when i'm resupplying grenades, i usually spam my supply pylon at random places, near ammo boxes, near end of the waves, or at any point in the map that I know that I will re-visit to, so I can preserve thermal clip packs when I really need them for maybe hack circle objectives.

I didn't have too much of a trouble against bosses, the only exception for me are atlases, not sure if I remembered geth primes being trouble for me, banshees is workable but risky esp running zero fitness and no cyclonic IV/IIIs. Atlases were kinda annoying because they have so much shields to chew down through with my build while using an arc pistol. With banshees when starts casting warp, use arc grenade on the floor so you still hit the banshee without trigger her power absorption bs, and then went up to her point blank, and directed the homing grenade at her foot    no point doing that but it works, it's better to go near the banshee and then bait out the scream and take full advantage while she's recovering.

For everything else that had straight up just armor, and not too sure if this was the case either because I haven't played ME3 MP in a while memory is hazy, if you charge up your arc pistol while having incendiary ammo on, throw an arc grenade, use homing grenade and during mid animation of the homing grenade you can release the charged shot and get a tech burst followed up by a fire explosion with the second split.

Fun kit to use, I remember one time I played her in platinum with some BSners on firebase glacier, I think firerider was one of them, and after the game, some not sure if all them but they switched to the demo girl, and can't help but imagine if it was "Oooooh Chris is playing demogirl, is she really that good!?!?!?! " it was a messy game but lots of grenades were thrown and we managed to get to extraction wave.

HamleticTortoise - the only build I use.

The battlefield is her playground. I go Arc Damage, Damage, and Armor Piercing on dem nades, and take my Omniblade and shove it all up in the enemies faces.

All I have to say is this - devastatingly fast AOE heavy melee.

I'm really sorry for necroposting here, but I figured this would be the best place for my question.
One of my friends I'm currently playing with told me a few days ago that DemoGirl's fitness passive isn't working. As I usually try to find some evidence for things like that, I thought people here (especially those magicians who did a lot of numerical tests) might have some data to back this claim - or to prove it's just one more in-game rumor or an excuse for unimpressive performance.
Could you please help me with this one?

On a side note, I've also heard that there are in fact quite a few characters whose passives aren't working properly (the most well-known ones being Turians with their TV passive which refuses to work off-host if you're the last one to join the lobby). Could you please list those you know about as well?

Edit: I assume all passives, frankly. The game works better in worse conditions than either of its BioWare successors, but a lot of data is still either lost or the coding is broken.

AKA Working as Intended ™

As for fitness, I believe DemoGrill Melee works better than intended, and almost always build as a melee/power hybrid kit skipping HM.

Melee demogirl works fine up to gold, is workable on plat, but not ideal. With melee selected in the tree she is quite weak, but a cyclonic 4 makes up the difference.

Her melee hits in a wide cone and ragdolls reliably. Tossing an arc first will strip Shields and stun.

I believe the fitness passives work fine on these kits and you just had the misfortune of playing on a host with higher fps. Uncapped framerate means you'll get hit by more shots while moving.