What started as a small comment went on to a lot more so apologies but:

Wall of text inc.

The brotherhood banner is something I nearly always have in at least one unit especially if I have added an independent character. If you have 5 paladins without a banner they have 3 attacks on the charge making 15 attacks. If you include a banner you have 4 paladins with force weapons making 4 attacks each for 16 attacks on the charge plus the normal attacks from the banner bearer. Add a character that also has the extra attack the banner has made back it’s points in the first assault. That’s not even mentioning the auto activation of the force weapons. Put them in a land raider or storm raven, DS or summon them using a Librarian to where you need them.

As Krunkumunga said the Purifiers psychic power isn’t a template weapon it’s actually better than that imo especially if you go up against those aforementioned 30 ork boys. In assault if you pass the psychic test every enemy model involved in close combat with a Purifier unit automatically takes a wound on a 4+ with armour saves allowed. Even if you only have one Purifier alive you can still use this against all enemy models in close combat.

A tooled up Purifier unit will also cost the same or less than a tooled up Strike squad and they all have 2 attacks not just the team leader (Knight of the Flame / Justicar)

I have one other reason you would take a Venerable Dreadnought. If you already have 3 heavy choices and want a rifleman dread (2 x T/L autocannons plus psybolt ammo) that can fire 4 S8 AP4 shots, hit on a 2+ and re-roll misses and the ability to make the enemy re-roll the damage result.

Speaking of re-rolls, Philld77 is correct regarding master crafted weapons I don’t know about scything talons but it’s possible the ability Darrell was getting mixed up with is the Hammer of Righteousness rule from Grand Strategy. Any units given that rule can re-roll any to wound (not hit) rolls of 1 for the rest of the game.

Another roll for a Techmarine is to equip him for combat with rad grenades (I so love these grenades) and have him join a strike squad or a unit of purifiers in a rhino. While in the rhino he can repair damage. Once out he has the harness weapons to use in the shooting phase and in combat he can use his hammerhand while the unit can use their psychic power for hammerhand, activating their force weapons or in the case of purifiers use cleansing flame. With the enemy at -1T and 1 or 2 hammerhands on the go the unit will most likely be wounding on 2+ and depending who you are fighting possibly even causing instant death without the need to activate their force weapons.