I've heard it about fast pieces. You can just play more like a machine, notes in time, and don't have to shape the line as much. So if a teacher doesn't think the student can emote, they might give the student technical piece that hides the weak area of note being able to play with expression.
I haven't really heard of that going the other way. If someone's good at playing slow expressive pieces, they might not have the technique (or desire) to play something fast. If it's not going to sound good, they wouldn't play it in the first place.
But for someone who's not emotional or expressive, I think it's possible to hide that with a technical fast piece and let them play like a machine. Not that that's good but, I have heard of it.