I was discussing that very same thing with my sister yesterday - I live in Junction, she lives in Mochdre and was leaving us to go home. I told her to be careful at that junction.
The lights on the A547 when approaching the big roundabout are NOT for the junction, they are for the pedestrian crossing that is a few yards before the junction - not that many use it.
Like you, I would expect anyone seeing the green lights to assume that it is their turn to join the roundabout but, as you say, there could still be traffic coming from either the eastbound slip road or from Glan Conway direction under the bridge.
I took a walk by there a couple of weeks ago and mentioned this to one of the workers. He told me he had already seen three bumps there since the lights were put up.
I also think some of the markings on the road are incorrect - as well as being extremely difficult to read due to so much of it, and you're trying to watch what is ahead.
Then, of course, there is the cycle lane through the middle of the roundabout. When I went for my walk I saw two cyclists using it - a father and young daughter. But I also saw two cyclists still on the main carriageway round the roundabout - they were lycralout types - you know the ones that complain of not enough being done to keep the cyclists safe - grrrr!