Besides the decline in user growth that Google mentions, here are the major problems I saw while using it:
* hosting a free Helpout meant dealing with constant no-shows [2]. I started charging $1/hour and then refunding it to folks who showed up.
* attendees of free Helpouts were rarely prepared
* the scheduling system was very limited (especially compared to Google Calendar)
That said, when everything worked, it was an amazing experience.
[1] https://helpouts.google.com/103350848301234480355/ls/a344f06...
[2] https://simon.codes/2013/11/07/google-helpouts-first-impress...