Imagine I give you two options.
You can take 10 Marines with 1 Wound each, 1 Attack each, and 1 Bolter each, for a total of 10 Wounds, 10 Attacks, and 10 Bolter shots outside rapid fire range.
Or, for about the same points, you can take 5 Marines with 2 Wounds each, 2 Attacks each, and 1 Bolter each, for 10 Wounds, 10 Attacks, and 5 Bolter shots outside rapid fire range, except they get 6" more range.
They both have the same number of Wounds and Attacks, and all other stats are identical, except the big ones have 50% the firepower.
>But they have -1 AP too
Okay, so they increase the chance of an enemy who has an armour save and not an invuln equal or better than that save of failing it by about 16%. So under certain conditions, not even every target, they have 66% of the firepower. For an extra 6 inches of range.
But wait, that's not all. If I shoot the Tactical Squad with 2damage weapons and kill two Marines, there is no overflow and they've lost 2 of their 10 Wounds. If I shoot the same weapons at the Primaris squad with the same results, they lose 4 of their 10 Wounds. So they're strictly worse against >1 damage weapons.
But wait that's not all. If the two units engage each other on an objective, and they each cause 3 Wounds to each other, the Tacticals end up with 7 models, the Primaris end up with 4 and one Wounded model. The Tactical Marines hold the objective, because model count is all that matters.
But wait that's not all. The Tactical Squad can take special and heavy weapons that give them more flexibility against more targets, in an edition where everything has universal split fire and plasma guns can be fired on a safe setting and melta is more effective against more targets and heavy weapons only get a -1 for moving and shooting, while the Primaris are stuck with their basic loadout.
There is almost no situation I can think of where the Primaris are better.