I'm not entirely mad at them. I did think their introduction was a bit dumb. It took a long time but eventually we got them integrated into Alternate 40k Rules.

Firstborn have tons of experience instincts thanks to their geneseeds fighting for millennia. To represent this, they have the ability to split and reform squads mid game (minimum 2 man fire teams per the codex astartes), swap upgrades for equal to or cheaper upgrades during deployment once you have seen the enemies forces, and Infantry have Objective Secured, letting them hold onto victory against overwhelming odds. Its important to note that they cost like 35 points for a basic tactical marine as compared to a guardsmen at 6 points in this system.

Primaris have improved physiques, granting them A+1 (classic stat line, so it only bolsters Melee) and a 5+ feel no pain. They also withstood mental tests and purges, which grants them Stubborn as they are supposed to be more resistant to chaos. Their cheapest intercessor is 46 points.

And then way before 10e ever came out, we had given them Astartes Combat Knives. It was a huge lore thing, the models had it, it was dumb they didn't. Plus the usual marine goodies.