Depending on how much one uses any of the other soul lines, as that decision point dictates things moreso than the ex skills themselves. The new ex skills are fine stuff to consider in a debuff resistant context (or, one in which your kamihime take care of all your debuff needs) once you've already committed to such and such soul.

Arthur (presumably you're Shingen-less if you're using Arthur): 20% chance of +20% crit damage for 2 turns is kind of weak, but it buffs all participants (exactly like how Arthur's atk buff targets all participants). Granted, it'll take coordination to get the most out of this in a raid/union fight. Alternatively, you're using Gawain for Ambush and want the crit buff for whatever reason (possibly being someone else's crit buff slave).
As for the self-only 40-50% vigor buff... ehhhh.

Joan (you've decided to commit your soul to defense): the single ally 100% damage cut on a 6 turn cooldown can be nice if you're looking for that ol' cover/invincibility combo to deal with random targeting overdrives. If you want that combo in one unit, use the ex with Roland. Olivier already offered a self only invincibility skill, but it's on a 8 turn cooldown. The new ex being 6 turns lines up with Roland's cover.
As for the stacking 4% def buff every 2 turns for up to 30 turns? Yea, that's long duration content only. I can come up with some uses for it for my own situation, but it's rather niche/specific to certain events.

Andromeda line (if you really want your soul to be a healer with maiden's prayer/chaos magic/res access): party-wide debuff block on... 8 turn cooldown? Very encounter specific. I don't think this would be used much outside of Medusa reprint.
Single ally endure buff on 8 turn cooldown? You can make a knock-off version of the cover/invincibility combo with that... if you're opting for the Andromeda line over the Joan line for whatever reason. But hey, if you've already committed to this soul line, it's an option.

Solomon line (why?... oh right, you want to do 0 turn stuff): Some big ability performance buff? Guess in case you're bringing in a nuke heavy party for those 0 turn shenanigans.
Self-only ability refresh? You're relying on yourself for said 0 turn shenanigans.

Siegfried (uhh, someone help me out here with what specific plans call for the Siegfried line): Self-only guaranteed double attacks for 3 turns every 6 turns? DPS-wise it should lose out to using Arthur's 3x-4x nuke every 6 turns (until said nuke rams into its 300k damage cap), but you do get the burst gauge buildup instead. Ehh, if I had plans for Siegfried, I'd probably use the other ex more than this one?
Party-wide soft damage cap buff. Well then. Presumably you're so strong, you're desperate for this sort of thing. That does not describe most of us.

Mordred (more for the lower power teams and/or for more conservative play; also, likely not to be used in debuff-resistant content): 6 turn cooldown orb eraser? It fits what you're trying to do if you're opting for Mordred in the first place. Fine option if I'm not going for Chaos Magic in the particular encounter. A-frame def debuff presumably taken care of by a hime.
Nuke based on # of debuffs applied? And no numbers given for the scaling yet? Bah, can't really comment on that beside the relatively high damage cap (700k?).

D'art: Eh, basically what you said. Mammon-like nuke's something nice when your ex slot choice comes down to 'damage'. And I skipped Snatch EX cause nobody cares about a Snatch version 4, right?

Hercules (when you still want the Herc axe def debuff and/or the one man army awesomeness): self only atk/damage cap stacking buff? Ehh.... does the cap stacking apply to full break?
A nuke that does 5% of current HP every 7 turns with a cap of 600k? You need an enemy with at least 12 million HP remaining to keep hitting that cap. And you'll need an enemy with at least 6 million HP remaining to get at least 300k damage from this to beat Arthur's nuke. And who knows where Overdrive caps at if the enemy's raging. By raw HP count alone, you're looking at lv 90/lv 100 raid type stuff to have encounters durable enough for multiple uses of this nuke to be relevant. But I don't know how those work. So I don't know if a Herc user can last long enough to get off multiple uses.
Alternatively, for leechers who want to at least contribute a bit of 0 turn damage, this is an option.

Morgan (you have some specific plan requiring party-wide berserk): first ex is... you give an ally a 1 turn cover buff? Ok, ok, you can put together that cover/invincibility combo if your team composition somehow has someone with self-only invincibility but no way of providing cover. Or combo with interceptor.
And the other ex is... give an ally a 10% pursuit buff for 3 turns on a 6 turn cooldown. Pure dps-wise, this sucks for the most part. I have some other guesses, but the issue is, I don't fully understand pursuit. Does a pursuit attack generate burst? Then this could function as 3 extra attacks worth of burst gauge. Does a pursuit attack apply effects that a normal attack would?

Shingen (the meta default): Encouragement. It's the usual meta choice.
That said, the +25% burst streak buff? There's a use for that when you don't need encouragement as much and/or you hit so hard, +burst streak is one of those things you dig up to try to circumvent soft damage caps. Again, that's a power level that does not describe most of us.