I've been using LabVIEW and I agree with you. LabVIEW has great hardware support which makes it really easy to get up and running collecting data.
But like you said, I've looked over some previous LV projects that have multiple copy and pasted sections, terrible design, etc.
As a software engineer, LV holds you back in my opinion. I don't doubt for a second that some really amazing things can be created with LV but ultimately it is extremely tiring/annoying to reinvent the wheel to accomplish things that would be extremely simple in almost any other programming language. This seems to come into play when you move past the simple act of collecting data and doing primitive processing.
Edit: The other thing I forgot to touch on was the presentation of the "code". With wires running all over the place and ambiguous function blocks you quickly find yourself wishing you could cozy up with a text editor instead. The main reason I don't think data flow programming will "take over" all programming is because ultimately I think its a inferior presentation for professionals.
You briefly touch on another point that is worth repeating. Source control with LV is terrible.