I am in doubt. Always used the Gulp to minify, concat, etc. But am in doubt whether to use the Ember-cli or continue using Gulp. For example, with Gulp I can use http://cssnext.io/ and that helps me in the projects.
Or I can use Ember-cli and GulpJS together? Then use the Ember-cli on the basic tasks that it already does and Gulp for more complex tasks?
If you're developing an Ember application, you should definitely use Ember CLI. It takes care of minification and the other tasks for you.
There's usually no reason to pull in other build pipelines. For the specific case of cssnext
for example, there are two addons, ember-cli-cssnext and ember-cssnext. The first one is maintained by an Ember core team developer, so it should be supported.