I would agree if it said "most" instead of "any". Euler angles are useful. Just be very careful with them. Convert to quaternion as soon as possible and convert back to euler as late as possible when needed.

However, you will never entirely avoid them because they are just too practical. Under careful constraints, they make code interfaces simpler.

This is especially important if you are to expose that code to a multidisciplinary team. There is no way you're going to teach an artist how quaternions work just so they can rotate their stuff around the up axis, when simple yaw-pitch angles cover 99% of their use cases.