So finally have a chance to look at the Horus Heresy rules, and gotta say
It demonstrates that GW do know how to make a good wargame system. Damn wild that this is being sold alongside 10th Edition.
Its a really timewalk back to 5th/6th for me, mainly in how 6th introduced and codified so common rules.
The army construction disallowing "legion soup" in your primary detachment, but allowing a secondary detachment to be a different Legion Is interesting, and I think does well to avoid having too many rules in play per game. So far as I see it, you literally just need the ore rules, and either flavour of Astartes books.
The "return" the Force Organisation Chart is welcome. Honestly I find the unlimited-ness of current edition to be somewhat decision paralysis inducing. Knowing there are minimums, and maxmimums to balance is just what is best imo.
One thing I'm saddened by is also how limited squad choices currently initially feel. I think this is more "old thinking" having developed as a gamer under 4th edition up through to 6th. Its the one area that feels most like current 10th, with squads basically having fixed loadouts except the sergreant.
It will be interesting to give this game system a spin when my friend and I get a chance, but will probably be a bit of proxy on my side as I'm building my Horus Heresy model to work with the 4th ed Chaos Codex first.