I just figured that DG was a massive screw up on the part of Shinra. During the wars, no would would question lots of disappearances, so they wanted to test things until they broke. Eventually, things stopped breaking so much, as the scientists improved their methods, but nobody told the kidnapping crews to stop kidnapping. So the numbers swelled, and suddenly there was a massive well equipped army in the basement. Killing them all will obviously be resisted, and would be impossible to do quietly. so there's no choice but to keep going and keep kicking the problem down the curb.
That explains the Tsviets. The Spiderbots, the helicopters, thousands-men strong Deepground (let's try to keep in mind that as big as it seemed in Dirge of Cerberus, it was but the mere sliver of what it used to be, they've been massacring each other day in, day out for three years straight with no new recruits coming in if Rosso is to be believed) every each single one of them outfitted with their own scifi combat suit aren't really a test anymore. It's just a massively better outfitted army then the one Shinra actually makes use off aboveground.
A big if. She's even crazier than the average Tsviet. I kinda assumed most of them were kept in Mako tanks, doesn't Vincent pass through some big rooms full of them?
I'm not sure it's better equipped. They basically have three models of robot, the spidercrab, the red saucer things, and the helicopter. Above ground Shinra have Scorpion Guards, Airbuster, the Moth Slasher line, the Sweeper line, Cromwells, those weird things that look like floating roses that cast fire, Mighty Guards,, the guns on legs, HundredGunners, Motor Balls, H2SO4 and everything else Hojo can cook up, and a lot of things I missed. DG seemed to have got hold of limited specs and just mindlessly made the same things over and over with occasional tweaks over and over.
I kinda got the impression DG thought they were more dangerous than they actually were. Vincent struggles against Rosso, but takes on Azul and Shelke in quick succession with little trouble. Nero's swirly black stuff is unbeatable, but he's not all that tough in combat -simple handgun rounds do him a lot of damage. And we never see non powered up Weiss, so he's hard to gauge. I wouldn't be all that surprised if keeping him chained was a way to control Nero, as crazily devoted as he is.
Sephiroth is a long term investment, it takes what, fifteen years before they'd be combat ready?