There are two I'm thinking are underrated in Crystalline Conflict, though that doesn't mean they don't need buffs.
Black Mage
Black Mages are deceptively tanky and due to Aetherial manipulation, they are a lot harder to kill than it seems.
-In order to make use of Black Mages kit, you *have* to use Burst and attempt to draw fire towards yourself. Burst is almost a free recuperate on yourself every 20 seconds and it deals a crap ton of damage. Warp to an enemy and Burst them, Guard if you think they're going to focus fire you, then warp to an ally at range.
-You need to be mindful of which of your party members is going to be fighting at range so that you have a general idea of who you can teleport to.
-Teleporting directly on top of the enemy range to escape the enemy melee is a viable option in a pinch. Then you can bounce back to a random teammate if they chase after you.
-If there's too many Line of Sight walls, you can always just alternate Fire and Blizzard on separate targets so that they're instant casts. This lets you setup an instant Paradox -> Superflare on one of them.
-Sleep is potentially 4 seconds of aoe CC. Get this on a cluster, then find a different focus target. Sleep can basically force a team to use an early purify without putting the Black Mage in any real danger.
-Black mage Limit Break is up nearly all the time. It continues charging while it's in effect so for half the game, you can shoot out instant casts left and right. During limit break, you can basically double weave recuperate so focus fire isn't even too big a deal.
-You do *not* have to cast at a melee that's currently attacking you. All of your spells allow you to weave in Recuperate so if you're not a focus-fire target, you can just stand there and keep hitting a priority target.
Bard
-On demand ranged Silence on an ability is incredibly good. MCH in comparison has to wait 40 seconds to cycle back to Air Anchor. Plus, Air Anchor is preceded by Drill, and succeeded by Bioblaster, meaning that MCH itself has really poor capability to capitalize on its stun to score a kill.
-Brings a lot of invisible power to the group due to the damage aura.
-Incredibly good at shot-calling, their ability to Blast Arrow-> Empyreal Arrow-> Silent Nocturne-> Pitch Perfect can deal a respectable 27000+ damage burst and lock down a priority target while also buffing your allies to help finish it off.
-Warden's Paean is the perfect tool to counter lockdown burst combos.
-As a general rule of thumb, you can generally ignore one person sitting on top of you and just weave Recuperate into your rotation.
-Repelling Shot should be used liberally on enemy melee to reduce pressure on your team.
-If needed, you can Sprint into Repelling Shot to catch stragglers.
-Repelling Shot makes Bard a bit more slippery than MCH. This combined with Paean makes you incredibly hard to actually focus down.
-You don't need to be the one standing on top of the crystal if your team comp is balanced.