I've only used the Steam-a-seam product (the "lite" one). I only had a problem with it once, when I was applying to a very large applique (life size female silhouette). The problem I had wasn't with the ironing portion, but the first step where you adhere the fusible to the fabric then pull off the first layer of paper. Because the applique was so large and "split" where the legs split ... the fusible part wanted to stick to the paper instead of the fabric in a few places. I was able to peel it off the paper and onto my fabric though. I did not have a problem fusing it with the iron once I had it on.
Part of my problem could have been that my fabric was heavily starched.