Not exactly trying to question whether it's a good/bad thing (as I'm aware that for many creators this might be exactly what they need), just trying to understand.
What might well be very hard is making a language where all abstractions and interfaces map to useful graphical representations; I imagine it's much, much easier to represent a JSON object graphically than it is to represent a piece of C code this way.
For instance, how would jumps look? Lines going from far-removed boxes, producing code that literally looks like spaghetti? Moreso, do you even want to graphically represent things like pointer arithmetic, will this be easier to work with? How do you represent variables, globals?
Maybe you would only represent course features such as data flow graphically, and hide finer grained details and code in "blocks" with labels and defined in- and outputs. You would still mostly write your code, but the graphical view might help you mentally model your program.
At present, FMJ is interpreted and runs on a virtual dataflow machine. This was a necessary step to ensure the computation model was implemented accurately. Compilation to run on a single processor would actually be simplified, however: there's no parsing or dataflow analysis required. Looking into the future, the question that should really be asked is whether languages designed to run on a single processor, or to execute the same instructions in parallel (SIMD), can be easily adapted to run on MIMD architectures. Do you really want to be using MPI?
I wouldn't want to do general purpose programming in them -- at least as they exist currently.
Different general-purpose languages are better adapted to different programming tasks, however, and Full Metal Jacket may in time find its niche. I wouldn't write a neural network in Prolog, or an expert system in C, for example.