The main problem, I think, is that it retains the vestigial features of being designed by electrical engineers who just refused to take any lessons from the 50+ years of experience we have had with programming languages.
---> Readability
You fall into the trap of thinking a flow diagram is an easier way to understand things. You are wrong. On any project that is not trivial, it's impossible to look at things and grok what they are doing or even what they are part of, or why that cable is going into that other system. You want to know what that cable does? Better follow it through the wormhole to see what variable it is! Realized you need a variable from somewhere else? Good luck bringing it over without a) creating a rat's nest or b) making it not obvious that it is coming from miles away. I could show you a simple for loop with an iteration variable, building up an array as it goes, and it'd take you a minute to get what it does. Readability sucks. Ignored lesson #1.
---> Refactoring
Fundamentally, the way you build code in labview tends to cause the problem where stuff gets added on all the time and it's all spaghetti. Granted, you can split things out into smaller modules initially (and interfacing them sucks). But there is no simple way to refactor things after you've written spaghetti. And because it's hard to manage external imports, you will write spaghetti. Ignored lesson #2.
---> Spatial Issues
The spatial paradigm is a culprit in many of these cases. Part of the problem is that it's hard to create spatial scope and visually import variables from elsewhere, all the while making the imports clear and easy to inspect all at once.
Don't even think of keeping things tidy, wires will be everywhere, and the auto-arranger doesn't work well.
You want to add code? Well, you should have left space for it in the first place! Now you have to move all your code aside to make space for the new stuff. Are you fucking kidding me?
So, you want to copy and paste. Good luck. All your wires disappear and since you haven't memorized what the code looks like, and you have to understand the code again before rewiring everything. Just like how you have to rename variables after copy-paste except dumber and takes much longer. This is why refactoring is hard.
---> Modern Tools
Version control you say? Forget about it! No diffs, no patches, no merging, no collaborative coding. Do NOT EVER EVER use anything but the master copy of the code (tm) or otherwise you'll end up with two versions and no one will know what is in which etc. Unit testing? Costs $1500 and is total junk regardless. Continuous integration? What is that even?
---> The UI
The UI has its own misgivings. Slow as balls even on a fast computer. Limited undo (are you shitting me?). Undo history used to disappear after you saved, maybe they fixed that. The window management is a mess. Sometimes when you try to quit, it says you made changes but you have no idea what changed. Good luck if you accidentally dragged some fucking wire somewhere so that now your robot flips out when reading gyro input. You can't know.
---> The Libraries
Extremely verbose and difficult to use. (especially math and strings).
Labview is just an all-round bad experience. Like I said, they've learned NOTHING from modern programming. And yet they tout this ignorance to you (and unsuspecting electrical engineers) with pride. This is just what I have off the top of my head. I don't even have my hate-list with me (yes, I compiled one to let off steam every time something was frustrating).