Seraphim are your go-to girls for zooming around the table eliminating pockets of resistance. Think dug-in heavy weapons or isolated squads: Flamed, shot at, and then assaulted to get rid of the survivors by virtue of hitting them a lot (note they ALL have two close-combat weapons). Melta bombs give them lethal anti-vehicle ability as well. Make use of Hit and Run to escape retribution and remember when picking your fights that there's Acts of Faith (Remember, the VSS being alive means you have a free Simulacrum Imperialis on the Seraphim squad!) to turn a fight from 'iffy/costly' into 'victory'. A Power Weapon on the VSS could easily pay off. So could the Eviscerator. All depends on how you use them
Note on Eviscerators: Both swords you get with the Canoness model are actually larger than the Eviscerators of the Repentia. Given that the Eviscerator is supposed to be grotesquely large, I suspect she's overcompensating for something (probably the fact that the SM now have bigger flamers than we do, which seems utterly unfair). That does mean you can easily use the chain one as an Eviscerator without anyone being the wiser.
About that second Canoness, if you have the points to make her useful and preferably a squad/transport to go with her, that's good. On the other hand, if you're looking to save a couple points or have another idea in mind, don't kit her out overmuch... Just basic weapons and Melta Bombs or an Eviscerator to make sure she can't be ignored.
She's worth 2 Faith points.
Once she dies, she's worth another 2.
For a total of four, at a bargain price.
At the risk of seeming totally heartless, Canoness Martha the Martyr has been a part of my smaller army lists for a while now