I think the Alicuberre drive is worse because it’s expensive science working on a project that violates known scientific principles in multiple ways. At minimum LLM has some good uses in business and to create boiler plate journalism. It might turn into something more eventually. Alibucurre drives can only end in futility because none of the stuff proposed can exist. Negative mass and negative energy are fiction. The theory of relativity closes off FTL travel and must do so to preserve causality.
In relativity, we assume that the speed of light in the vacuum (which, in the mathematical and physical sense also means there isn't anything for it to be relative to) is the same in every frame of reference. This is what Michelson–Morley experiment measured, and it is remarkable. It is unintuitive, because if you were throwing someone a ball at rest vs from a moving car, the moving one would have a different velocity. But that doesn't happen like that with light. Regardless if you're stationary or moving away or toward me at 0.9c, we'll both see both c as c. What we can't do is accelerate things to the speed of light if they were sub-luminal for the same reason we can't decelerate super-luminal objects to the speed of light: it requires infinite energy. By the way, doing that to light in either direction is equally difficult and for the same reason. This, however, doesn't mean that FTL travel is impossible. To be precise, relativity describes it. It doesn't describe how to jump over the light speed barrier, not how to treat the light speed as some unbreachable limit of the universe. Why FTL could happen and how it relates to mass? Where do I even start, Higgs fields? QCD binding energy? Affine spaces? Maybe, but I doubt you'd read it. No, I'll just restate this one more time: it doesn't have to forbid or preserve a thing. It can't, on its own, explain how we go from sub-luminal to light speed or super-luminal down to light speed, or how to jump over the speed of light in either direction, but it doesn't prohibit faster than light travel. It literally describes time travel in both directions, not how to move over discontinuities. Which, mind you, could be purely mathematical artefacts. I'll give you one better: the problem with wormholes isn't that we can't figure out negative energy or transit stability, it's that we wouldn't be able to make them connect specific points in spacetime. It's for a similar reason why you wouldn't be able to steer Alcubierre bubble as we understand it, not why they couldn't exist.The theory of relativity closes off FTL travel and must do so to preserve causality.
I don't think they're fashionable, it's that I accept the fact we're observing only one part of GR without having to blast string theory out of every co-tangent orfice bundle. Here I jumped onto the 'closes off' and 'must', since relativity a) doesn't show to do the former, and b) doesn't care about or explicitly enforce the latter. It's a bit like every time I hear someone talk about theory of everything only have them add exclusionary clauses and asterisks each time I open my mouth.
RANT ON Perhaps "fashionable" is the wrong phrase. How about "orthodox?" The basic issue, as highlighted by this article, is that some ideas are acceptable to papal doctrine and some aren't. Those ideas very much follow the Planck maxim of "science advances one funeral at a time." Scientists capable of creativity tend to tuck their crazier ideas into short stories, which is hella fun. Scientists incapable of creativity tend to tuck their crazier ideas into weird little papers in neglected corners of academia, which is hella tedious. Max Tegmark even has a ratio; in order to keep tenure he does one hare-brained paper per 19 normie ones. Analog Magazine used to publish speculative fact. It was usually awesome, tooled for a lay audience and written in a conversational tone that was invariably thought-provoking. On the other hand, modern speculative papers are dry AF and couched in all sorts of plausible deniability which means a lack of scientific rigor is easily hidden (see above). So what we're left with is blogs discussing idiots when we used to get Sagan arguing that there wasn't life on earth (based on current scientific methods). So now we're at "is it vaguely thought-provoking? give it to the crazies" and "does it reinforce doctrine? Make Neil DeGrasse-Tyson talk about it on Good Morning America." Even if it's nutrition - especially if it's nutrition. And the thing of it is? It's the science fiction that pushed the envelope. It's the science fiction that the scientists read. Enrico Fermi wasn't known for his ruminations on little green men and yet the autocomplete on everyone's phone for Fermi isn't "-on" it's "Paradox." But god help you if you try and push a boundary or two. Nobody reads science fiction anymore, it's all Hunger Games ripoffs and Chinese propaganda. So we don't talk about the black spots in mars rover tracks. And we don't talk about the platinum-iridium wire Avi Loeb dragged up from the ocean. But ZOMFG the UAPs RANT OFF