While me, and i'm sure, many other Black Mages are happy about the umbral soul changes, the fact is, the class has many issues beyond that, that can't be fixed by a simple potency buff.
Before I list the gameplay problems we fact, one thing has to be clear.
Black Mage is not a burst job.
Increasing the potency of Xenoglossy further, with three stacks of it, is going to force us to use them in the party buff window, restricting our movement options, and generally not feeling fun to play.
With that in mind, I shall move into the gameplay issues.
1.Astral Fire both lasts too long, and feels too strict.
This is probably my largest gripe with DT BLM. With the addition of Flare Star, our fire phase is reaching the point where our High Thunder starts to fall off, to speak nothing about Thunder 3, which is 3 seconds shorter than HT. On the latter point of the statement, Fire Paradox being instant, actually gives us 2-3 less seconds to work with, as we have to deal with the recast timer. It doesn't replace Ice Paradox in its movement tech, as the trade off of a lessened AF timer never feels good. This is also the same issue with using F3P to refresh.
2.Flare Star is not a good capstone reward.
I can understand the conception behind Flare Star. A big, meaty finisher to end your fire phase. You'll likely get it every time you do a fire phase, and it has pretty decent VFX to boot. However, in gameplay it falls flat. Despair is already a commitment, as it's (at base sps) 0.2 seconds longer to cast then F4. Flare Star shares that same length, and that's a problem. Back in EW, many times I'd find myself refreshing thunder, or keeping enochian up by the skin of my teeth, because of another cast in the phase. To add another throws off the timing that was established in ShB and EW, and further serves to make the job feel awkward and janky.
3.Thunderhead, both a noob trap, and a bad replacement.
Again, I can understand both the conception of Thunderhead, and the removal of Sharpcast. In most scenarios, you'd only use Sharpcast to keep a charge of Thundercloud (the buff that let you instantcast thunder) in your pocket, and not much else.
However, Thunderclouds replacement with Thunderhead brings about a few issues. The DoT length is the same as our proc, making it impossible to not clip it on ST scenarios. With Thunderclouds duration being 40 seconds, (as opposed to DT's 30), this is a rather baffling and harebrained oversight on the devs end.
Another issue is in the very nature of Thunderhead. If you're a new player, I feel like you would press the glowing button that pops up everytime you switch phase. It's just not good UI design.