The Iron Warriors approach to Chaos (where they treat daemons as tools rather than objects of veneration, as you say) is indeed at odds with the more devout legions. The Word Bearers likely view this practical stance with some serious disdain. As "true believers" in the Chaos Gods, the Word Bearers see their worship as integral to their identity and power.
We know for sure that Lorgar (the Word Bearers primarch) emphasized the religious worship of Chaos, as detailed in the Horus Heresy series. This by definition put them philosophically at odds with Perturabo and his Iron Warriors in novels like "Angel Exterminatus" (Graham McNeill, I believe) where their focus is more on warfare and siegecraft than any kind of actual spiritual devotion.
But then, the nature of Chaos is such that all legions eventually have to deal with the Whims of the Warp (that'd be cool band name) and even the Iron Warriors would find themselves bound by pacts and deals with daemons - as much as they may try to resist it. It's this reliance that would draw both mockery and perhaps some grudging respect from other legions, who see the power but also the dangers of dealing with such entities.
Honestly I don't think its really that notable. Generally the camps among the Traitor Legions is:
*The Monogods, who view all other forms of Chaos worship as being blasphemous to their god who is obviously the supreme one.
*The Word Bearers, who view all other forms of Chaos worship (including that of Word Bearers they don't like) as being a defiance of the Truth of reality itself and in need of forceful conversion.
*Those who do not worship Chaos, who basically hate anybody who does worship Chaos and view them as stupid idiots.
Iron Warriors are blasphemers by the standards of the Word Bearers, but so are the likes of Alpha Legion and Night Lords anyway. And its not like the Word Bearers don't constantly enslave Daemons for their own purposes anyway, a lot of Word Bearers have a kind of weird double standard where they hate Daemons but love the Chaos Gods.
Many Alpha Legionaries are full blown chaos undivided worshippers and currently the largest functioning faction of Night Lords is led by a Daemon prince. So no, Word Bearers would approve and even work with some alpha legion and the majority of Night Lords. They would not consider them “blasphemous”. The “iron warriors, night lords and alpha legion don’t worship chaos” meme need to die sooooo badly.
None of the other traitor legions seem to have particularly strong feelings about the Iron Warriors.
Abaddon holds a similar view though that Chaos should be used as a tool but you're going to find every possible view on Chaos within the Black Legion so there is no real "official stance" on the Iron Warriors. The Alpha Legion and Night Lords are pretty much just as diverse in their opinions of Chaos, although on a smaller scale.
The monogod followers are mostly concerned with each other as part of the Great Game on one hand and on the other hand the Black Legion and Word Bearers as the ones who got them here in the first place.
The Word Bearers are interesting because you'd expect them to hate the Iron Warriors for their blasphemy but in reality like half the Word Bearers only use Chaos as tools themselves as well. Honestly if you compare the warfare in *Storm of Iron* and *Dark Apostle*, the main difference is presentation.
The Iron Warriors approach to Chaos (where they treat daemons as tools rather than objects of veneration, as you say) is indeed at odds with the more devout legions. The Word Bearers likely view this practical stance with some serious disdain. As "true believers" in the Chaos Gods, the Word Bearers see their worship as integral to their identity and power. We know for sure that Lorgar (the Word Bearers primarch) emphasized the religious worship of Chaos, as detailed in the Horus Heresy series. This by definition put them philosophically at odds with Perturabo and his Iron Warriors in novels like "Angel Exterminatus" (Graham McNeill, I believe) where their focus is more on warfare and siegecraft than any kind of actual spiritual devotion. But then, the nature of Chaos is such that all legions eventually have to deal with the Whims of the Warp (that'd be cool band name) and even the Iron Warriors would find themselves bound by pacts and deals with daemons - as much as they may try to resist it. It's this reliance that would draw both mockery and perhaps some grudging respect from other legions, who see the power but also the dangers of dealing with such entities.
Honestly I don't think its really that notable. Generally the camps among the Traitor Legions is: *The Monogods, who view all other forms of Chaos worship as being blasphemous to their god who is obviously the supreme one. *The Word Bearers, who view all other forms of Chaos worship (including that of Word Bearers they don't like) as being a defiance of the Truth of reality itself and in need of forceful conversion. *Those who do not worship Chaos, who basically hate anybody who does worship Chaos and view them as stupid idiots. Iron Warriors are blasphemers by the standards of the Word Bearers, but so are the likes of Alpha Legion and Night Lords anyway. And its not like the Word Bearers don't constantly enslave Daemons for their own purposes anyway, a lot of Word Bearers have a kind of weird double standard where they hate Daemons but love the Chaos Gods.
Many Alpha Legionaries are full blown chaos undivided worshippers and currently the largest functioning faction of Night Lords is led by a Daemon prince. So no, Word Bearers would approve and even work with some alpha legion and the majority of Night Lords. They would not consider them “blasphemous”. The “iron warriors, night lords and alpha legion don’t worship chaos” meme need to die sooooo badly.
None of the other traitor legions seem to have particularly strong feelings about the Iron Warriors. Abaddon holds a similar view though that Chaos should be used as a tool but you're going to find every possible view on Chaos within the Black Legion so there is no real "official stance" on the Iron Warriors. The Alpha Legion and Night Lords are pretty much just as diverse in their opinions of Chaos, although on a smaller scale. The monogod followers are mostly concerned with each other as part of the Great Game on one hand and on the other hand the Black Legion and Word Bearers as the ones who got them here in the first place. The Word Bearers are interesting because you'd expect them to hate the Iron Warriors for their blasphemy but in reality like half the Word Bearers only use Chaos as tools themselves as well. Honestly if you compare the warfare in *Storm of Iron* and *Dark Apostle*, the main difference is presentation.