And to be fair that is pretty boring and lazy, but still, I don't see why anyone should care that much what minis someone else wants to use.
This is an interesting point. Playing a game of Necromunda is a “social contract”. This phrase is mostly intoned when speaking about playing in the spirit of the game and being a good sport, but IMO there is also joint responsibiiity to create something of a visual spectacle.
Many players put a lot of effort into ensuring their gangs are beautifully modelled, fit into the background and are fighting over stunning terrain.
Of course not everyone is equally as talented or cares as much about the modelling side of the hobby, but there certainly are some basic requirements (figures that fundamentally fit the setting and represent what they are in the game, appropriate weaponry, an attempt at painting and basing), and limits beyond which I would say people are well within their rights to care quite a lot what minis their opponent uses.
If someone turned up to play with 12 miniature bananas with 40k weapons blutacked on, you could certainly play the game but by doing so that person would quite possibly be ruining the game for the other person. This could be just as damaging as being a poor sport.
While a gang of identically armed Catachans isn’t quite
as egregious (and let me state for the record if it wasn’t obvious, that I’m in no way suggesting the OP was inclined to do this), they certainly don’t fit the setting (largely due to the identical weaponry) and so I could understand if someone saw such a gang being lined up against them and had at least a small concern about the models being used.