Is it unethical? I think that before long, we won't think that cloning humans is unethical. The reason is that the moral stances start sliding when the idea becomes more and more well known and normalized, in for example fiction.
With GE though it's less messy because there's definitely a second person at the end of a GE'd baby (that's the whole goal of doing it after all) that's completely separate from the mother/parents.
The more similar genes are, the easier it is for a single pathogen to kill all those that share those genes.
But who cares about clones, engineered humans would be much more useful, potentially.
So, say "clones" instead of "twins".
Putting aside, um, "assembly", what makes feeding, housing, educating (at least to worker-bee levels), etc. them cheaper, easier or faster than the normal way of getting new productive humans?