Whats wrong with merle? (genuine question)
On it's own? Nothing, it has some quirks to keep in mind when you are breeding it in any breed. (highly recommend reading the paper by Mary Langevin and her team on it if you want to go into the pure genetic details)
In border collies specifically however 9/10 merles aren't bred for work and haven't been bred for work in many generations giving you high energy, often unpredictable dogs who are sold to the pet and beginner sport market. I owned and loved one of these dogs, so this isn't to judge people who have found yourself in this position. (It's slightly more common in conformation lines, but those aren't where my knowledge lies.) Now ALL of those color bred merles go back to the dog Astra Mars who was a decent working sheep dog in the UK but is now known to produce epilepsy and is a major red flag for epilepsy risk in a pedigree and/or BH Rowdy who was a mostly cattle bred dog who is was known to produce several health issues but more importantly behavior issues which is seen pretty heavily in his progeny.
I've been actively looking for a merle dog to add to my program that does not go back to either of these two lines for over two years now and I've found two studs, one of which I still suspect goes back to Astra Mars.
Merle isn't unique in this aspect, brindle is also a gene that in color bred lines almost always goes back to the dog Deep Sea Kreechyr who himself was extremely neurotic, died of a genetic cancer, and had several pups die from the SAME genetic cancer. Brindle IS found a bit more common in working lines than merle but as sable is very rare in working lines, can be almost impossible to see (Templehall Cymru is a good, high profile example of a working brindle. He is genetically tan pointed with brindle)
So that's the overview on why a lot of (working) border collie people tend to have poor associations with the gene as the dogs don't work and often are of poor mental and/or physical health. But again, that's not the fault of merle as a GENE but rather how poor breeders have taken advantage of the breed and the popularity of the color.