Copied from the B&C.

Disclaimer: I'm relatively new to Custodes.

After reading about the shield-hosts, watching batreps with Shadowkeepers, and a few lists that test out that selection, I have to say I'm pretty disillusioned with Grim Responsibility and Shadowkeepers as a whole.

Here's why - the strategem subtracts 1 from the strength of the attack, in either melee or ranged, correct? So let's take this list I came up with as an example (in spoiler) and let's say I got hit with something equivalent to lascannons on my 4 man brick of Vertus Praetors. The lascannon which is S9, goes down to S8. All well and good, however here's the problem. Both instances of the attacks, they would still be wounding on 3's as the bikes are T6. Same story goes if if said lascannon went up against my sword and board Custodian Guard (S9-8 against T5). Still would be wounding on 3's. Seeing how most of our units are in the T5-6 range, why would you run this shield host?

That being said, with this in mind, and with the notable caveat of running Vehicles (T8 or 7) in Shadowkeepers, or up against DG which subtract 1 from your units toughness, and (there I can see a use for Grim Responsibility as they would be wounding on 4's instead of 3's!!), what makes that Shield Host selection actually good? The WL trait is interesting granted, and the Gatekeeper relic in conjunction with Superior Fire Patterns stratagem is also good, but honestly that's probably it. I'm just failing to see why it's the default go-to-choice in nearly every Custodes list I saw played when you have other alternatives such a Dread Host (WL trait = spicy, as well as the strat), or (on occasion) the Emissaries Imperatus with the Emperor's Hand Strategem. 

Side-note: In this list I provided, what Shield Host selection/WL trait would you recommend? The WL trait/Host selection is temporary.

Patrol Detachment 0CP (Imperium - Adeptus Custodes) [101 PL, 10CP, 1,997pts]

Configuration

Battle Size [12CP]: 3. Strike Force (101-200 Total PL / 1001-2000 Points) 

Detachment Command Cost

Shield Host: Solar Watch

Agents of the Imperium

Vindicare Assassin [5 PL, 100pts]

HQ

Shield-Captain [6 PL, -1CP, 100pts]: Captain-Commander, Guardian Spear, Strategic Mastermind

Shield-Captain on Dawneagle Jetbike [9 PL, 175pts]: Auric Aquilas, Hurricane Bolter, Warlord, Superior Creation

Troops

Custodian Guard Squad [7 PL, 149pts]

. Custodian: Sentinel Blade, Storm Shield

. Custodian: Sentinel Blade, Storm Shield

. Custodian: Guardian Spear

Custodian Guard Squad [7 PL, 149pts]

. Custodian: Sentinel Blade, Storm Shield

. Custodian: Sentinel Blade, Storm Shield

. Custodian: Guardian Spear

Custodian Guard Squad [7 PL, 149pts]

. Custodian: Sentinel Blade, Storm Shield

. Custodian: Sentinel Blade, Storm Shield

. Custodian: Guardian Spear

Elites

Aquilon Custodians [22 PL, 375pts]

. Aquilon Custodian: Lastrum Storm Bolter, Solerite Power Gauntlet

. Aquilon Custodian: Lastrum Storm Bolter, Solerite Power Gauntlet

. Aquilon Custodian: Lastrum Storm Bolter, Solerite Power Gauntlet

. Aquilon Custodian: Lastrum Storm Bolter, Solerite Power Gauntlet

. Aquilon Custodian: Lastrum Storm Bolter, Solerite Power Gauntlet

Vexilus Praetor [6 PL, -1CP, 120pts]: Guardian Spear, Impregnable Mind, Ten Thousand Heroes, Vexilla Magnifica

Fast Attack

Vertus Praetors [16 PL, 340pts]

. Vertus Praetor: Salvo Launcher

. Vertus Praetor: Salvo Launcher

. Vertus Praetor: Salvo Launcher

. Vertus Praetor: Salvo Launcher

Vertus Praetors [16 PL, 340pts]

. Vertus Praetor: Salvo Launcher

. Vertus Praetor: Salvo Launcher

. Vertus Praetor: Salvo Launcher

. Vertus Praetor: Salvo Launcher

Total: [101 PL, 10CP, 1,997pts]