I believe they can be very effective, if you understand where the participants are as far as their readiness to change is concerned.
One to six week challenges, in my opinion, are short enough to an individual to set and reach SMART goals they set for themselves if someone walks them through how to develop SMART goals.
Great question and suggestion!
I think that they CAN motivate the participants because the ‘time certain’ element requires them to stay focused, to remain consistent, and to put forth their ‘best’ or at least a good effort because time is working against them – the clock is ticking. 🙂