...But the EMH could replicate new brain stem nerves for Ptera in "Emanations", restoring her back to life.
Though I once again remind you and everyone involved to take Voyager technobabble with a bucket of salt.
Remember that in "Ethics" Dr. Russel's technique is complicated enough that Doctor Crusher considers it more of a dice roll than solid science; in the end, the only reason it worked at all is because Klingons turn out to be really
hard to kill.
IMO, the whole "genetronic replication" thing is the exception that proves the rule why living tissue can't be replicated. I would theorize that the technique only worked because it gave Worf's central nervous system something of a scaffold to rebuild itself
, using the replicated tissue more as a suture or a splint than actual replacement tissue.
There obviously are degrees to what can be done, not strict categories. New brain stems are okay, even with the resources of a damaged and stranded starship, but new lungs for Neelix are not.
I'm of the opinion that "new brain cells" in this case is actually a bit of cybernetics. Probably the things being replicated were ARTIFICIAL brain cells, something similar in function (but vastly less sophisticated) to the microcircuitry in Data's brain. Like Picard's artificial heart, but on a microscopic scale.
A culinary obsessive might insist on 100% lifelike steaks and get what he wants, but at high cost; somebody else would accept the same taste and texture as achieved by different means, through replicating something that's only 20% like real steak but features suitable "cheats" to enhance/"restore" the taste and texture to match natural.
Stop me if I'm wrong, but isn't this ALREADY the case with real/artificial food substances? Most people I know can't tell the difference between real and imitation crab meat if they're not eating them side by side. Same with regular milk vs. soy milk (in fact most people think soymilk tastes better than skim milk by an order of magnitude). We already have lots of techniques for reprocessing raw protein from a variety of sources into something that tastes like something else, but real cooks--my wife, for example--can always tell the difference.