Internal ffmpeg aac encoder is still experimental and tends to create broken AAC streams at times. The other options:

A) libfaac / libvo_aacenc are pretty bad in terms of quality and have problems with multi-channel audio B) libaacplus only encodes up to 64kbps in less supported AAC+

So Nero and Apple are pretty much only quality free audio encoders for AAC available right now.

I don't think AAC encoding is a strong point for ffmpeg.