So Rug and I played using the 3k list he posted above (with 8 missions, 3 dominions and two lightnings).
I deployed poorly, keeping all 8 missions together right at the start - they weren't going to be engaging turn 1 so they didn't need to be intermingled like they were so it was a mistake that the artillery punished me for.
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First thing that happened though was the garrisoned, intermingled Dominions all surged forward to engage some sentinels, leaving the formation fearless, on the side of a forest hidden from the Imperial Guard, and within striking distance of practically his entire deployment zone on turn 2.
The manticores fired, damaging three formations of Missions but failing to break any of them.
I then moved the damaged missions forward - burning activations basically to try and draw out his Leman Russ and Mech Inf companies. Despite the pounding from the Manticores the Missions were - while filled with BMs and down a few rhinos and sisters - still meaningful threats in turn 2 if they were able to clear their BMs. The entire center of the board was filled with sisters in varying states of morale.
His sentinels got in the way, aircraft flew around a bit and tried to clear them, his Leman Russ came forward and fired into some of the unactivated missions. A Mech Inf Company decided to have a go at clearing away the intermignled Dominions and failed fairly spectacularly (though one of the dominions formations was broken and left with one fearless stand).
By the end of turn 1 the Guard were down 2 sentinel formations, a mech inf company was broken and down to about half strength and trying to flank the sisters. Otherwise his formations had no BMs. Manticores were on cooldown. The center of the board was packed with damaged (but largely reduced BMs) missions and three unscathed, intermingled missions with a palatine and a cannonness standing opposite his intermingled mech hq and leman russ.
Turn 2 starts with the Dominions combined engaging fearlessly into the leman russ company, killing a few and breaking, they swept behind trying to encircle the massed clump in the middle. Retaining and engaging again the three unscathed missions barrel into the mech hq and leman russ. The dice betray me here but I win by a single point due to outnumber. The guard are down to some Manticores that can't fire, some thunderbolts, some hydras and two warhound titans while I have two lightnings and 5 unactivated missions all close together and all within engage range of something worthwhile.
We don't do turn 3 because of time and the more or less inevitable conclusion.
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Overall thoughts
1. the stats, weapons, etc. on the formations feels right, I don't think the statlines or weapon load outs need to change for these units
2. faithful...it's powerful but using it is always a choice with meaningful pros and cons, early on in the turn you want to use it but once the BMs start coming down and you start retaining and prepping targets etc. the risk increases.
3. Why would you choose Repentia when you could take Dominions?
4. These formations are too cheap which makes them too spammable, perhaps bump their price by 25 points so Missions are 250+transport and Dominioms are 200+transport (this army would be losing 275 points which would be a mission and two immolators to help reduce the activation advantage).
5. The other formations in the list don't feel as competitive - the Chapels, the Cathedral, the Inquisition stuff especially - to reduce the spamming of the core sisters I'd up their price but also make the other formations more attractive to take so that the default response to a list building problem is not 'when in doubt, take more missions'. Someone taking Chapels means they aren't taking another mission. Plus they'd have a reason to put some cool centerpiece and unique models on the board!
6. Celestians don't feel all that useful, I don't know what I'd do with them in the list, perhaps find some way of making them a more attractive option, making the BTS more points but also more potent.
Rug had some interesting ideas about how to change the way you buy Celestians and Retributors, he may want to share those ideas here?