In my experience:
1 - if the leader hits autorun, /follow breaks
2 - if there is any unevenness in the terrain (even slightly), /follow breaks. Sometimes, even if the color of the terrain changes, it seems to break.
3 - if you go on a bridge/lift, /follow breaks
4 - if the lead character is moving at all, /follow breaks
5 - if the sun is up during the day, /follow breaks
Ok... so the last 2 may be exaggerations, but basically /follow simply does not work.
I have never had a char /follow another for more than about 3 seconds. Ever!