They can recreate the look of Luke’s hand, seamlessly, when it wasn’t on fire, it was completely gone, but Vader’s charred body right away is put into a suit.
Well, if Vader’s head was cut off, I’m pretty sure they could replace it seamlessly with a robotic head and artificial skin, but healing is completely different than recreating.
There is only so much even super-advanced space technology can do. When Luke comes out of the Bacta tank, he still has some injuries that need to heal naturally, maybe Vader’s injuries are just too severe.
And if you take the prequels into account, women still die in child birth, so the Star Wars medical equipment doesn’t seem as advanced anymore.