Although one of the downsides (imo) is you have to play the chord progression for EACH new song you play. You cannot save or store them (on the old version - new one allows 12). So for just drums it might be okay, but if you want to add bass this is a restriction that could be important.
Also (again just imo) the drum sounds are a bit machine like in sound and oepration.
One of the video descriptions available on YouTube indicate the looper function can be used to store up to five instrument groups. Each instrument group can be broken into discrete parts such as intro, outro, chorus, etc. It seems that you can use the pedal as inspiration to build a complete song.