Are there any objective advantages to a left-handed in playing volleyball?
There are some advantages for right-hand side players (setters and opposites): in the same way that it's easier for a right-hander to hit down the line from the left-hand side of the court, it's easier for a left-hander to hit line from the right-hand side of the court. In particular, if the setter wants to do a surprise hit rather than setting the ball, they almost have to hit it left-handed - there's no real way to create deception with a right-handed hit. There's a good video of left-handed hitters here - as you'll see, almost all the hits are from the right-hand side rather than the left-hand side (Uroš Kovačević being the exception).
Here you can find a research by Psychonomic Society from 2011 "On the advantage of being left-handed in volleyball: further evidence of the specificity of skilled visual perception", which has the following conclusion:
[...] findings indicating side-specific visual perception support the assumption that the orientation of action presentation has a considerable influence on athletes’ anticipation performance in interactive sporting situations. Even at the early developmental stages of perceptual expertise, highly specific adaptations occur to the cognitive system, and after years of practice, these adaptations become highly specific (Ericsson & Lehmann, 1996). [...]