They are hard to make (physically hard) -- you really have to push the cables in hard. I have pretty strong hands, and if I make a few of them, it's hard on my hands. The wire/post inside the plug that you impale the cable onto is like a half-inch long. You need to push the wire in, until you feel it hit the back wall of the plug.
You have to use a new, sharp razor, and DO NOT distort the wire. If you need to press down hard with your blade (and you squeeze the wire out of perfectly round shape) you can have problems. A sawing motion is better than squeezing with your blade. Your cut should be perfect. Flat on the end, and a round shape.
You can test your (built) cables right away, by inserting them into your amp input (don't set it super loud) and handling the other end of the cable with your fingers. If it makes a noise, the cable is good. If not, trim the wire at one end and try again.
After you do it a few times, you can usually tell if you are making them correctly, or if you'll have a problem.
These are best used for established boards rather than setups that you are constantly changing around. Is your pedal setup practically written in stone, at this point? Is everything velcroed into place, or ready to be velcroed? Then George Ls are for you.
Only time I had a problem was buying used plugs (here) that had the finish worn off. I couldn't get those to work. Somebody on Talk Bass sold me some worn out George Ls