No.
Turnbull was doing fine at Celtic until he was injured. By the time he was fully fit again, the new manager had built a different style of team without him. Cardiff was probably a bad move, though.
Miller is highly rated, but that doesn't mean a 19 year old is ready to be a starter in a team in a new country. Arsene Wenger said it took about 6 months for a new player to adapt to a new team in the same league. Longer if they moved to a different country. You can a take a look at teams like Liverpool this season to see how some of their, undoubtedly talented, new signings have struggled.
I understand the reasoning, but Miller joining Udinese without a proper pre-season in Motherwell or Italy might have been a rare misstep.
While you can never guarantee that a youngster will flourish in later years, Miller looked about as close to guaranteed as you can find.
Max Johnston was rated too, and he won a cup and two league titles after he left. So picking the right team and right country also plays a big part.