Some pitching coaches clearly add value. Don Cooper with the White Sox is one obvious example - who has been able to work with guys with odd deliveries and has taught the cutter to pitchers and changed their outlook. (perhaps a reason Nieves was a good candidate to begin with)
Nieves would be hard to blame for the pitchers woes - but it also does not seem like he has provided consistent value-add to guys coming at the major league level. Yeah, there is clear PR with this move, but hard to be too worked up in either direction.
One thing that makes sense though is, as Farrell's area of expertise, I have to assume he is looking at pitching coaches who match his philosophy or can be taught it. And if the pitching coach is not doing some magic, he can fill the gaps in. Or at least you'd hope so.