Well… I still make dumb mistakes so I have lost my Bolt Thorwers turn 1 vs wood elves and lost them vs ambushing high elves BUT the second time I have used them they did keep eagles hidden for most of the game and when they ame out bolt thowers shot one down.
Elves also deployed their infantry blocks and cav regiment away from them so they did a good job. Hitting on 5+ at long range means You have 2/3 chance to hit and when we talk large targets at close range this is a very solid weapon hitting on 3+. Also if Your enemy spend a whole game hunting them with ambushers then You already won.