This is really disappointing to hear, as I too saw some YouTube videos and have some experience with FDM 3D printing on a smaller scale, I got rather excited at the idea of being able to produce professional looking colorful, LED signs using something I already know and use (3D printing).
In regards to the outdoor durability issue, you mentioned printing with PLA and ABS, but I've found that both of those are indeed terrible for outdoor use.. but PETG however is GREAT. It's much more UV stable, and has a much higher glass transition temp than PLA, so it shouldn't get "soft" sitting in direct sunlight unless it's enclosed inside a parked vehicle with the windows up on a HOT summer day. Sitting on it's own, outside, or attached to the front of a building it shouldn't get nearly hot enough to start to get soft or warp at all.
I had considered the time factor, and I understand it would be a lot faster using more traditional methods (bending sheet metal, or milling them out) but I assumed it could be something I could start and run overnight, and after a few nights worth of printing, end up with the shells I need to produce a complete sign.
I also went on the assumption that I would only be producing smaller letters/signs anyway (<18" tall), and I wasn't planning on trying to compete with other methods like bent aluminum while trying to produce huge, 20" tall channel letter signs anyway, so the additional printing time isn't as much of a drawback for me, especially when considering 3D printing makes it possible to do things that would be extremely difficult if not impossible for standard bent aluminum channel letter signs to compete with, like utilizing multiple extruders to produce shells in different colors.. using a translucent filament to produce a "stripe" around each letter that light can pass through, so not only are the letter faces lit from within, but each letter has a lit-up "stripe" around it, making it almost look like they have a band of neon around them.
Another exciting prospect of 3D printing is the ability to vary the width of each letter/elements so they are wider in back, giving each letter/element a dimensional "debossed" look (also called the "pyramid" effect).
And because you can see all of the sides of each letter/element at once when you're looking at it straight on, If you incorporate thin translucent bands at different points throughout it's depth.. you end up with a really cool lighting effect that looks like your letters/elements are circled by bands of colored light!
I REALLY want to believe this is a viable method, SO badly! But I can see what you guy are saying about it taking forever.. but if you COULD tweak the process enough to be able to reliably let it print overnight, don't you think this could be a nice way for a smaller shop to be able to expand their offerings, and produce really unique, stunning-looking LED signs? And there's always the possibility of scaling up production by incorporating multiple 3D printers (farming) as well.
I hope this "bump" will encourage some other users to chime in with their experiences (good or bad).
Thanks guys.