yes when playing as the Gauls (postpatch , veryhard/hard ) I was able to destroy the 2 main briton army's in an ambush.and several other armys
for me it worked best when I used units that were good at ambushing/hiding ( easy when playing Gaul ) and well then It's pretty random if your ambush fails or succeeds, in my game it worked every time, not 1 failure , so I guess I was lucky.
well don't hide ur units near watchtowers (sometimes you get attacked when in ambush ''mode'' on the campain map, so it's probably a must that the enemy doesn't see you hide in the woods, other wise they still see you (camp map ) and attack , which is weird..
hoped that helped