Hi Calum,
I always split my scenes up because I always assume they will need to be at some point in the future and it is often easier to do this up front than it is to retrofit in later.
When you have multiple people working on a project it is a good idea to split up the data into multiple scenes to avoid conflicts. This way different people can make edits to different scenes without having to resolve conflicts. Even though the scenes can be set to save in text format, conflicts can still be a nuisance and are not always easily resolved. This is also true for a single person developing where they decide to work with branches and end up editing the same scene in different branches.
Additionally, the size of the launch scene dictates how long you sit on the splash screen for. Therefore keeping this initial scene as small as possible is good practice to help your game load in as quickly as possible, this is especially true when running on mobile. As soon as you are past the splash screen your own code is executing and you can control exactly what the player sees as you load in the rest of the games resourses.
Lizi