I did various bits of freelance work on VVVV projects a while ago. Often writing C++ or C# code to interface with VVVV. Even when I was mostly stuck in the guts of the C++ bit having a test rig in VVVV was a lovely way to interact with and test code.
As far as I remember it’s absolutely simplicity to create a node that contains custom C# code (double click it and an editor window pops up). I definitely remember doing that for bits of logic that seemed easier to express that way.
I saw people build very cool stuff in it back then. If you’re building interactive visuals the “always live” way of working seems to allow some pretty fast iteration. Definitely saw things built in it that maybe would have been better built in other ways, but that’s just natural when you have people highly skilled in one tool or language.
See for example Grasshopper for parametric architecture, or the node system in Blender.
OFC maintainability of a spaghetti node system is borderline impossible so there are constraints and tradeoffs.
Accessibility to non-software engineer domain talent is the main attraction imho.
That said, you just have to treat it like code and build up good patterns and styles to keep things tidy.