Haha, I like that line about God giving the talent to only a few to cut down on the competition!
I use a half butter, half shortening, whipping cream and milk icing. It doesn't crust so much as is sets. That seam line is always a challenge. Honestly sometimes I literally pinch with my fingers all along that line. Other times I use my little fondant roller or fondant smoother on an angle to take dare of it. Some folks use the pop bottle routine that Sewsweet uses for her Faux Fondant.
Some folks have better luck with icing the sides first, others with icing the top first the sides last. So maybe try reversing whatever you do normally.
I agree the larger the icing spatula the better, Some prefer the straight one, others the lifter/bent one.
It does come eventuallly though. Then the trick becomes trying to remember what the heck you did this time differently and wondering if it was only to tease you into thinking you finally figured it out!
A folded piece of parchment or waxed paper all along the edge and pinching it as you go. I hve even hear of someone taking a sheet of craft board the same size as the cake. Put a fold in it and place it over the top of the cake with the fold lined up to the seam and pinch.
Until you get that line right, though, choose to make your top border in the same colour of icing as the top and side of the cake. The same colour won't jump out as much and draw the eye to the imperfect cake edge. Also instead of trying to pipe your border all along this seam, go in slightly along the top and keep your cake at a level where you can see that you are piping in a straight line. I hope that makes sense. What I am trying to say is as long as you pipe your top border perfectly straight, even if your seam line isn't, it won't be noticable. But if you follow a crooked seam line, it will just make it pop.
A vertical dropped shell border is another good camoflauge. You know, the one where you do each shell vertically from the top of the cake down, resting on the side? Well as long as you train your eye to make these in a straight line, they will hide this seam line. So will a really large star type of tip.
And keep practising!
Hugs Squirrelly Cakes