Hm, part of my consulting agreement when I did Puppet was that I could re-use my own set of Puppet modules (which I never did open source). I shared the same set of Puppet modules among many clients and never had issues with "minefields" because of my explicit requirements.

Therefore, I think this comes down to how you design your Puppet modules.

Personally, I design my Puppet modules much in the way that Jamie Windsor designs Chef cookbooks and talks about in the ChefConf keynote: "The Berkshelf Way".