I just heard from a Korean game developer, ttmayrin, who implemented Screen Space Decals in his engine and he loves it. The only problem he had was how the decal border was too obvious because my implementation used in Space Marine lacks any smoothing effect.
He added a fade-out effect based on the height and kindly shared his code on his blog. His blog is in Korean, but the code and screenshots below ("stolen" from his blog) should explain itself very well.
It's a very simple and cheap way of implementing it and definitely could be one of the decal types artists can choose.
Please note that SSD's projection box has unit length, centered at 0, 0, 0.