I've heard that they are, but I'm not convinced, especially if you have a class that get instantiated from a base module.
I think I'm saying that right.... instance variables based upon module instantiation ruby...
I'm still a little unsure though. Apparently there are some issues with it, but actually it doesn't really matter because I was only concerned with regard to the way I'm setting up Sinatra apps.
Nowadays I use a shared base and thankfully the settings configuration interface works as I need it to.