I made several of those afghans, in various plaids but not real tartans. One made for a baby about 25 years ago is still being used by his mother as a TV blanket! They really are cosy and warm.
The thing which could, and did, go wrong was losing or gaining a loop while doing the horizontal strpes, which is realised only while doing the weaving, far too late to correct.
They really can be beautiful blankets - I've recently made one for my newest DGS, but was limited in size by the amount of Pinguin "Mist" I had in the right colours.That was the original yarn used, and looks best, but I have made one in a cheaper yarn, which I've used for many years. Pinguin Mist is no longer obtainable, here at least, but I have a few balls kept especially for making a couple more lap-rugs, in a couple of colour combinations. Keeping the right amounts of each colour for the stripes is the biggest problem!
I'm sure one could make a real tartan using this method, but it would be even more essential to keep the number of loops correct, so very time-consuming. The method with the afghan hook sounds really ingenious, and must be beautiful.
Joyce in RSA.