-Grants an accuracy bonus to melee and ranged attacks from behind an enemy.
-Each additional increase placed into Ambush increases melee and ranged accuracy by 3, up to 15 with full merits.
Having an accuracy bonus from behind a mob could be some what nice under certain situational aspects of the game, but under 90% of the things you would do you would be better off with merits into something else.
An accuracy bonus from behind a mob is somewhat redundant on THF since Sneak Attack and Trick Attack provide almost a 100% accuracy bonus.
And as far as melee strikes go, I have plenty of other ways to enhance my accuracy to the point that one of the other Tier 2 merits would easily benefit me more to merit them over Ambush.
You have Assassin's Charge which give you an automatic Triple Attack every 15 min(or less with merits).
Feint, which gives a huge drop in the mobs evasion for a small period of time.
Aura Steal, which allows me to attempt to steal a buff from a mobs every 15 min(or less with merits).
Any of these other 3 Tier 2 merits would be much more valuable to go into than Ambush.
What I wanted to propose was, what if you changed it to provide an attack bonus, critical hit or critical hit damage bonus instead of an accuracy bonus? At that point it would easily make it comparable to the other 3 merits category choices, you could tweak the attack bonus so it's not too over-powered. Anything but accuracy would probably make it a lot more worthwhile than it is at the moment.
Now you don't need to use my idea exactly, it's only a thought. But I really do believe Ambush needs a revision of some kind, because at the moment it is a very mediocre job trait.