Mmm I guess it is about timing. ... As ever, and how much terrain is being used. If only two are behind the enemy, only two trolls will be able to come out... And if the terrain is some distance apart the solo trolls are easy pickings even for smaller companies of archers hanging back. I get tempted to start bringing ambushers out in turn 2 and 3, wheer maybe it should be 3 and 4.. when the enemy has closed and committed along the frontline and hasnt so much to patrol the rear.
Don't get me wrong I love the idea and love the threat of the tactic, but in two attempts, despite winning, I hadn't mastered the art of the ambush...
Those archers hanging back (or even better, siege engines) are such lovely targets for an ambush. When 3 coys of Rangers suddenly find themselves fighting 4 coys of Feral Uruks or Burhdur or some Wargs ambushing using Burhdur's battlehost, they are going to run (or die). Siege engines can't even turn around to fire at you for a turn.
"... Telchar wrought it in the deeps of time."
-On Andùril, The Lord of the Rings