These are questions from a layered product PoV (WildFly Camel in my case):
(1) Is it still safe to rely on the traditional feature packs?
Yes, they are still fully supported for now. Although they are not used anymore to build the default wildfly distributions. There is a command line argument though that allows to build the dists using the legacy feature packs.
(2) When will the time come that we'll have to switch to Galleon?
I'd say this has to start happening now. Ideally, by the end of the summer layered, deployed and other products that are using feature-packs should be using the new feature packs. The sooner we start the move the more time we'll have to adjust and identify what needs to be fixed.
Feel free to ping me for guidance here or in Galleon room on hipchat. I used to have the docs in my personal repo on github, now that the project has moved to its official repo I need to upload the docs somewhere official as well. Currently the docs are in the corresponding modules in Galleon project  and Galleon plugins .