Want to say, I find the sergeant for eliminators quite badly pigeon holed. Ether he takes the carbine and becomes "shoot better" sergeant with defence angle or takes a sniper rifle and never uses the buff. Just how I feel with them btw, would of likely been a better option to add a tax to additional eliminators.
However I will make a comment on the pose thing as I debated this with a GW store guy (and a good friend too, good fun to play against). The models without question look much better than the old firstborns, we can't debate that. They do have better poses which is great, can't argue that considering all firstborn marines are generally a variant of standing there at attention or aiming (or if you get vanguard/assault marines, a variant of running or jumping) which only had so many poses you could get with the main variance being angle of how high he holds his power sword or how far up he holds his boltgun, so that part isn't really debatable imo: the new Primaris poses are much better and for your troopers and other general units this is fine, you don't need each one striking a pose like the Ginyu Force!
However the main remiss is the degradation of kitbashing being a thing. One of the cool things about the Firstborn sets of marines is if you buy a Sternguard box, Assault Marine box, vanguard box, Tactical box, devastator box and a commander box (or any combination of any of those) they are all full compatible with each other. Chest pieces can be swapped around, arms traded and armaments given to other guys. Wanted some Combi-weapons? Sternguard box will see you right. Wanted melee? Vanguard! Just wanted a bunch of pistols and chainswords, assault marines.
The ability to make us of parts across the range is what made them so special and fun. It is fun to think about the possible combinations of pieces you can put together for firstborn marines and its fun to experiment. This didn't hurt any of the boxes ether to note, each one was still functional and gave you the unit you bought. Granted the downside of the older kits is a major one: you needed multiple same boxes to use optimal load outs for certain squads (devastators are a big one since most loadouts are best done 4 of the same). (quick aside: why can't tacticals take heavy flamers...I could get some major sales of the sternguard box for GW to salamander players if they did that ).
So there is trade off in all directions. It is arguable that Firstborns had too many options which made any kit for them impossible to make complete (though if they updated the tactical box to include all their options I would be impressed) while so far all Primaris boxes except for a notable exception have all been complete packages from when you buy it. Each one gives all options they have, exceptions being early release versions like shadowspear and (possibly) indominatus. The notable exception I speak of is Intercessors who have their sergeant options locked behind chapter specific upgrade sprues which imo is just bogus and feels back.
It leads to my gripe about primaris being half baked with their release. Why isn't the Primaris box 5 sprues, with the 5th being melee stuff for the assault variant instead of having a whole other box for what is effectively a niche variant squad. If they had the option for jump packs, I would then agree with it but they don't so just why? All just so they get running legs? Bit weak. But oh well, we are still waiting on the Gravis Captain to be released and now he is technically OOP...so we now have a model within the current codex that isn't made anymore? What happens now, we move a him to legends? Keep him in the codex...but that violates their "we don't make it, we don't have a sheet for it officially" policy...so what do they do now?