O'Toole has improved from second round Super Draft pick to a regularly useful player.
Gaby Pereira flourished while playing for Nick.
There are other players who have not done much, or even regressed like Talles.
It's a decidedly mixed record.
I would agree that it is a mixed record - but believe the record shows that Nick has had more success developing defenders than attackers.
Successful development - Gray, O'Toole, Haak
Success, but small sample size - Gaby, Jones
Unsuccessful development, stagnation, or regression - Magno, Fernandez, Ojeda, Mijatovic, Bakrar
Alonso Martinez is 26 and doesn't truly fit into the development analysis. It is more opportunity analysis. Not sure if credit for that goes to coaching or front office or both, but it is a different category.
Perea has been with the team for essentially one season. Again, opportunity more than development.
Santi is, in my opinion, also outside this level of development analysis (despite his age). He crosses eras and reached contribution (and salary) levels in a different manner.
Even though opinions may differ on success and categorization, it appears that development of defenders has outpaced development of attackers. Nothing wrong with developing fullbacks and defensive midfielders, but there is a disconnect between money spent by the front office (all attack) and player development (all defense).
I would also note that development has a geographical split, with North American players seeing more improvement than those from elsewhere. I don't know if that is relevant or coincidental.