I believe the substrate you are thinking of is crushed walnut shells, not coconut fibre. Crushed walnut shells can pose a lot of problems, as the little pieces have a lot of sharp edges that can cut up your snake if ingested. I use it way back when it came out, but have since switched. To answer your question, I never experienced any problems with mold.
Originally posted by Invictus
Sure, they will bury in a particulate substrate if that's all they have, but let me ask you this.... if I buried you in cocnut fibre, would you be able to breathe? Guess what... they can't either.
Coconut fibre is not at all dense, but rather light and fluffy in its dry form. Perfectly suitable for burrowing species. I would use it any day over shredded aspen, as well as it doesn't pose any serious risks of impaction like almost all loose substrates do.