Recently, our team start move out templates to BEM-methodology. After that we faced with several problems, i.e. absence of instruments, which automatically produce short variant of classes in production automatically. Using of this two technologies has not discussed in internet yet. It is strange, because this tool very powerful and useful.
Ember does not care about your CSS, you can use whichever technique you like to organize it - BEM is one of those, and it can be a good choice for complex apps.