It's not the type of glue, it's the quality of the joint. Make every joint fit as tightly as possible and you'll use less of whatever glue you use.

Filling gaps with either epoxy or thick CA is a no-no. Balsa is lighter than either one. A good tight fit means you use a very thin layer of glue and you can use whichever glue you need for the particular application.

A gob of epoxy isn't the way to get a servo rail to stay in place. A carefully cut balsa gusset with a thin film of epoxy, CA, aliphatic resin, Ambroid or whatever will be lighter and stronger. Firewalls? I use nothing but epoxy but with a good tight fit and balsa triangle stock to increase gluing area. Tail surfaces, actually I've changed to using mostly CA here and have never had a failure.

Dave