I think the main thing to avoid in fractal-based nebulas is streaks or patterns that appear too uniform (straight lines, even curves or parrallell forms) or sharp (like that spider's-silk effect you sometimes see) some of those are visible away from the stars, but yes - getting a soft and natural looking fractal is hard, hence why I rarely use them in my own nebulae, when I do, I often use many and just keep piling them on in different blending modes, merging, adding more and I can make onl 4 or so fractals go very far and make one great nebula
Have fun.