Merging source code line by line is a solved problem. Merging visual code/graphs/graphics is often simply impossible. Also versioning and simply showing diffs become difficult problems with visual programming. That is why visual programming will never scale beyond small toy projects maintained by a single developer.
That said, I agree that visualising your code base might give additional insights. However that is not visual programming, that is code visualisation.
Maybe it's not impossible but just quite difficult? I use Houdini 'Vops' sometimes and I could imagine a tricked-up diff could be made for it (especially since it translates to vex) but you're certainly right that it's a hard problem in general!
Well visual programming is standard in Unreal projects and they definitely scale beyond toy projects with a single developer. Although Excel is the most popular visual 'programming language', the second most popular is surely Blueprint.
"if you connect to source control within the editor you can at least diff blueprints to compare changes. though it's currently not possible to actually merge them."
Yes, but you don't need merge to scale up things like game projects. You can just carefully partition the work between people. Perforce supports file locking for this reason. And, a lot of merge conflicts in software are thanks to the use of symbolic naming.
This is very true. Line merges do not always work well. There used to be a tool called SemanticMerge which was able to handle merging code cleanly even when the code had been refactored. It saved me quite a bit of work a handful of times (before it was taken away because the company needed a value add for their paid version control software).
That said, I agree that visualising your code base might give additional insights. However that is not visual programming, that is code visualisation.