This is one of my big problems right now, I progged ex1 and had a rough time with it on BLM but I felt like it was doable, but ex2 I just gave up trying to learn the fight on it and just switched to RDM until I melded a bunch of spell speed and could do the mechanics more comfortably. Even after clearing ex2 well over a dozen times on BLM now, I cannot get over how the overlap of fire phase's new strictness/trying to save stuff for burst window/doing the fire/wind blade warp mechanic which happens EXACTLY at the same time as the burst window feels so incredibly hectic in a not great way, and I love the fight. Almost very single time I'm either like "ok, lets focus on burning through xenos for burst and staying alive for the mechanic" and then I look over and see I've dropped enochain, or I focus on the mechanic and keeping AF up and I don't maximize burst damage.
The thing that's getting me is that I did not ever feel like this during extreme or the few savages I did back during endwalker, and I was not a nonstandard player at all (I thought the concept was cool but it felt unintuitve to me/i hate using tranpose because its hard wired into my brain as the move I use when I've messed up.) Obviously dawntrail content is already more intense, but I feel like if they're not going to design the fights to accommodate for black mage, they should have at least tried to add things to BLM to be able to deal with more movement out of the gate without a crapton of SpSd melds besides Retrace (which is nice, but completely useless in constant movement mechs like ex1's Rain of Feathers) and another Xeno slot (which ends up just being saved for burst anyway). Don't get me wrong, I think the challenge of figuring out how to best use leylines/triplecast in a fight is part of the fun of the class, but currently I think it's too punishing if you ever do mess up, getting back into the flow while still doing the kinds of mechanics in ex1 and ex2 is very trying.
I like BLM's sustain damage style, its what it's built around and I don't want them trying to rework it to be just like other burst DPS, but I feel like most of these changes to the job weren't made with enough consideration for what the new content is like. I did not need the thunder damage changes to give me a new thing to fret about and worry about dropping thunderhead or the DoT itself since you're discouraged from refreshing too early now, or a new slowcast spell that punishes me from missing even one fire iv because i tried to adjust for a mechanic. I don't think its impossible to add stuff to BLM that makes it more capable of dealing with these kinds of fights without undermining the core of the class too. I'm kind of bummed that the devs either didn't realize (or just didn't care) that nonstandard rotations existence was a symptom of Black mage's flexibility, and by haphazardly trying to neuter it they took part of BLM's ability to adjust for mechs with it.