Quick thoughts and notes on Apple GarageBand...
The most obvious minor shortcomings, which are unlikely to bother many people unfamiliar with more professional audio editing systems, are: no way to fine-tune the settings for the Ducking function, and lack of sufficiently fine detail in editing.
A problem of length
There's a really silly potential pitfall, which is this: the maximum length of any GarageBand project is 2,000 bars, in whatever time signature and tempo you have selected. If you are working on a podcast, and simply accept the 4/4, 120bpm default when creating a new project (and why should you even think about it... it's not a musical project, after all), that will give you a maximum running time of about 1 hour and 9 minutes. So if you have imported a load of audio clips to GarageBand, and find that you just can't play beyond around 1m9s, that's why. (Quick explanation... we don't record or edit the basic show in GarageBand, as Zog just likes using AudioDesk, but we do use it for creating the enhanced version of the podcast.) To fix the problem, just lower the tempo, which will increase your running time - but note that any chapter markers that you have already created will be moved, as they appear to be fixed to the original bar/beat positions rather than to a time.
We did find on one occasion that GarageBand failed to correctly mixdown and export a finished podcast project, either to iTunes or to the hard drive. It spent around 10 minutes working through the progress bars, and apparently mixing down and then converting to an AAC file, but at the end... no file was actually created and saved.
Update: this bug appears to be specific to a particular version of GarageBand under OS 10.3.9.