because the ink layer is so thin, these circuits have an incredible amount of resistance, even with really fat lines.
I have one of these pens and it is a fun novelty, but not useful for practical circuits. The metal content isn't high enough. It's better applied to art projects and for kids teaching where you can show them a lot of circuit concepts in a very visual manner. That's where I felt this product would excel. If you drag a lead to a lightbulb over a 10" line, depending on how thick it is the bulb will be completely out either in a few inches or with a fat line over the whole length.
A version where you squired a lot more of the material with a thickener out of a mustard type squeeze bottle would get you some more functionality. But it's fun for the kids.