Apart from the fact it's a perpetual Juve generating machine? Seeing as you can potentially get a new Devotee after each game, you equip them with the cheapest gun (auto pistol), and move from place to place without upkeep costs, without House Gang tax, and the whole mob could still scavenge, meaning the bigger your gang got, the more you could scavenge, and if you stopped at specific places through the ruleset, you also didn't need to pay the 3 creds each to avoid starvation. Meaning lots of models = pure profit = more models = more profit = gang so big no one can actually stand a chance of making them bottle. Abused sufficiently, you had to be a really good player to deal with it.
You could win quite a lot of your games against them, but the Redemption had every advantage in the Post Battle sequence and no drawbacks whatsoever. I mean there was literally no point playing anything else.
Underhive is a joke of a system, fixing what didn't needed fixing and generally either making the non-house gangs even better, not remotely changed or actually worse. I think the only rules I like of the new edition were a lot of the fan-submitted stuff (like Nik Jakos' bestiary entries) but of the official, only Skavvies were much of an improvement, and even then I thought they were borderline broken.
The only good thing about all of Underhive is the High Impact rule, which I use as a House Rule alongside the original ruleset which we use. Now that GW have removed Underhive from their Website there is no reason to support that edition whatsoever, as it is inferior in every way to every other version of the game. Either play the community edition, written by the Necromunda Community who intend to keep the game alive and not just make a quick buck on a damp squib (which is what Underhive is, and I'm being generous with that) or the original ruleset, which is flawed, but most of the flaws can be fixed with a few pragmatic and simple House Rules. I don't agree with Underhive's house weapon lists, I don't agree with removing catch fire (just make catching fire on 6s rather than 4+ like the Community Edition does it) and removing so much of the useful content from the Hardback just to resell a brainless and uninspired, half-baked, mistake ridden knockoff is a detriment to what made Specialist Games good in the first place, and I'm half inclined to suspect that the awfulness was deliberate.
So yeah, as you may guess I'm not a fan of Underhive in general.