There are no inherent benefits to Unity over Ren'Py, but a decent chunk of people who code for a living, will already know Unity where as they will have to learn Ren'Py, and that is no small thing.
One advantage to Unity is that (if you have sufficient skill) you can design it to work the way you want rather than being forced to do things the way Ren'Ry makes you, the disadvantage is that there isn't an underlying code base to work with, so you have to do ABSOLUTELY everything yourself (unless you can crib some code of someone else using Unity, but this can carry some risks of it's own, particularly if code is poorly documented, and lots of code is, because the people who write it seldom do it for others and they already know how it works). So it can take longer to code things in Unity (at least until you have a sizeble code base of your own which does everything you want it to), and if it is a multi-person project, every member has to work within the framework laid down by whoever created the code base. If the person just sat down at their keyboard and started coding (and this is really common), their code will usually be a mess of work arounds and jury rigging, where as if you designed everything from the start it would be much more elegant and efficient, of course to do this you have to know absolutely everything you intend to do before you start, and then have properly analysed the flows of data, and most visual novels are passion projects (or cash grabs) so they just don't do this. Of course if you have a few visual novels under you belt, that can really help as you (hopefully) won't make the same mistakes you made the first time, and you can use the tricks that your learned from your first attempts to build better projects in the future.
Ultimately some people will just prefer unity because they know it, and because it is a multi platform frame work, I would imagine it is easier to port to other operating systems such as Android or IOS (but I have no experience of this, so I could horribly wrong on this)
One advantage to Unity is that (if you have sufficient skill) you can design it to work the way you want rather than being forced to do things the way Ren'Ry makes you, the disadvantage is that there isn't an underlying code base to work with, so you have to do ABSOLUTELY everything yourself (unless you can crib some code of someone else using Unity, but this can carry some risks of it's own, particularly if code is poorly documented, and lots of code is, because the people who write it seldom do it for others and they already know how it works). So it can take longer to code things in Unity (at least until you have a sizeble code base of your own which does everything you want it to), and if it is a multi-person project, every member has to work within the framework laid down by whoever created the code base. If the person just sat down at their keyboard and started coding (and this is really common), their code will usually be a mess of work arounds and jury rigging, where as if you designed everything from the start it would be much more elegant and efficient, of course to do this you have to know absolutely everything you intend to do before you start, and then have properly analysed the flows of data, and most visual novels are passion projects (or cash grabs) so they just don't do this. Of course if you have a few visual novels under you belt, that can really help as you (hopefully) won't make the same mistakes you made the first time, and you can use the tricks that your learned from your first attempts to build better projects in the future.
Ultimately some people will just prefer unity because they know it, and because it is a multi platform frame work, I would imagine it is easier to port to other operating systems such as Android or IOS (but I have no experience of this, so I could horribly wrong on this)
