3/5 stars. Very fluffy and such a great conversion opportunity if you ask me. There is an obvious negative here though.
Background.
The marines are making use of alternative weaponry. This might be because their usual ones are tainted in some way, or more likely because they have no way of replenishing their stocks, or have been forcefully disarmed of their bolt guns and similar. They are now wielding lesser arms that would ordinarily be found among regular mortals!
Rules Evaluation.
Models must exchange their bolt guns, and bolt pistols for lesser weapons on a one-for-one basis. Heavy stubbers come for every three models, but otherwise you can mix and match whatever weapons you like within a squad on a model by model basis. Interestingly, you can upgrade before being made to swap which provides a way for veterans to still take plasma weapons for instance which is nice. But my thought here is that you might as well go all out for the fluff - and this is doubly fluffy in small points value games (which I am very fond of).
There are some positives though. Pistol (X) substitutes in for Assault (X). Similarly Assault (X) comes in for Heavy (X). And Rapid Fire is just Pistol (X). This enables some good rates of fire and movement to be had. Be let's be honest here, the hit to the S value of the weapons is a massive issue despite the rates of fire that might be available. So yes: this is a very fluffy rules for those who love the narrative play style, but ultimately it isn't hyper competitive for ranged combat.
Combinatoric Analysis.
The Eternal Vendetta: ⭐️⭐️⭐️⭐️. 4/5 stars. I like the combination despite it being circumstantial. I can readily imagine a group of marines being deprived of their weapons by a different legion that causes resentment.
Panoply of Old: ⭐️⭐️. 2/5 stars. Short ranged weapons are fine here. But long range weapons are ones you will have to give up. So not really great.
Only in Death Does Duty End: ⭐️⭐️⭐️⭐️. 4/5 stars. A bunch of sub-standard armed marines who seek atonement works very nicely in my view.
The Spoils of Victory: ⭐️⭐️⭐️⭐️⭐️. 5/5 stars. An almost ideal combination in my eyes. Of course they will loot others - they need the upgrades!
An Eternity of War: ⭐️⭐️⭐️. 3/5 stars. Crazy with desperate ranged weapons works. But that's about all I can say here. Average overall.
The Flesh Is Weak: ⭐️⭐️⭐️ 3/5 stars. Another average combination that sees desperately armed robots take to the field. Perhaps some deranged Forge World adjuncts?
The Legacy of Nikaea: ⭐️⭐️⭐️ 3/5 stars. Desperate psykers? Sure! These are obviously ones who ignored Nikaea and have escaped imprisonment by more hated Primarchs perhaps?
The Broken Helix: ⭐️⭐️⭐️⭐️ 4/5 stars. Mutants and clones who were deprived of their weapons seems to make a lot of sense to me in a narrative way. Go for it - with either variant.
In Disgrace All Are Equal: ⭐️⭐️⭐️. 3/5 stars. This works: they are already disgraced and their use of lesser weapons marks them out as such I guess?
Pride Is Our Armour: ⭐️⭐️. 2/5 stars. I just don't quite buy the premise that a bunch of veterans like this would be so poorly armed. Sure, it can happen, but there are better narrative choices to my mind.
The Taint Of The Xenos: 0/5 stars. Yes, you could get Xenos Halo Blades, but the ranged weapons results in a crazy mix of desperate weapons and xenos weapons. Okay, I just read that back and it could actually totally work since your force is just picking up every weapon that they can. Its just very extreme. Not a fan.