I think your group might play Necromunda very diffrently from mine, and maybe most people's. Because, well;
Plus the bigger the Crusade becomes the worse their chances of winning a game become.
This sentance goes against just about all my Necromunda experiences ever. The only real downside of having a bigger gang is income, either because of the income filter or because of starvation. The boosts of having a large gangs is that you have a lot more meatshields, a lot more punch in CC, a lot more chances to pin the opponent, etc. etc. ad naseum. And Redemptionists gain more money the more models they have, and they don't use the starvation rules, and can even get free guys when you capture someone or someone dies, so the usual downsides of large gangs don't apply. As a result you get a gang that's bigger than all of the other gangs and profits from growing bigger.
Exterminators are expinsive and have to be attatched to weapons 50 + 15 + 15 X 17 = 1370 credits. Impossible for a starting gang and unlikley untill late campaign.
Ah, bad grammar on my part. I didn't mean to imply that all 17 guys had exterminators. It was probably "just" six or seven of them.

And 15 points for a one-shot flamer isn't expensive in any way, shape or form. For any other gang it costs 25 points and demands that you give up a roll on the rare trade. For each.
On a side note it's totally possible to get 18 Devotees with Massive weapons and exterminators for under 1000 points, by the way. I don't know how good that would be, but I wouldn't want to face it.
A normal gang can field hordes too if the want and will probably come off better for it too but nobody does because they are seduced by the Heavy Stubber of Plasma Cannons they can get.
Why would house gangs come of better? They're severly penalized for having to large gangs which is the main reason house gangs often have 9 or 12 models starting out.
My CC group got creamed against our Escher in their last meeting and I lost my Eviscerator too. Redemptionists are not broken, they simply offer a different play style. If you can't pin a Redemptionist Devotee horde outside of flamer range then start playing Delaque.
Redemptionists are not unbeatable. But they ARE much harder to beat than ordinary house gangs, simply because they can afford to bring more, relatively well-equipped, models than the opposition has the resources to deal with. It's not because they have a diffrent play style, because they don't. All CC-oriented gangs benefit from having lots and lots of models, it's just that most of them can't afford to have very many and instead equip the ones they have as well as they can. Redemptionists, on the other hand, gain money exponentially to how many gangers they have, which means that they can expand their gangs fast and faster and
faster as their numbers increase.