Only rogues have instant damage this ammy allows any class to have instant damage.
Yes it helps all classes but for bosses you simply stand close to the spawn.
And yes it dosent teleport you but pierce isn't a bad damage type and you can lock mobs with it. As an instant skill it is basically giving a rogue like skill to anyone. Not all rogues have the ammy, most well equipped ones prefer/have other gear.
Lets forget the inconveniance of running up to the mob the moment the skill is ready, and focus on rangers (second?)most powerful skill, longshot. Nevermind by the time rangers use longshot, rogues have already used ss, qs, ammy skill (based on your saying its instacast), and are loading their next skill, but long shot had a requirment that goes against the ammys skill requirment, Distance. Either the ranger can start up against the boss to use the ammys skill, then run back to use long shot, or start off back to use longshot, then run in to use ammys skill. Also, id imagine a ranger wouldnt have more 2-3k hp for a strong bossing build, which makes them vulnerable to bosses aoes. Running in could mean death (idk how much AoE damage bosses do, but it seems reasonable to assume that it could kill them). Ignoring the previous point of death, its a completely inconveniant skill for rangers to spend 2m on, and rogues would still be using the skill more effectivly. If anything, the skill widened the gap between rogues and classes that fight from far off, not shortened.
Also, about your saying that there are better ammys for well equipped rogues, that means that the ammy they put in its stead leads them to doing more overall damage then these ammys would, right? In that case, wouldnt rangers have that same type of ammy? What good in closing the gap would these ammys do for rangers then if this ammy doesnt even effect the gap? (Meaning that assuming there is a gap between rogues and rangers damage, and they both have ammys that increase overall dps in a bossfight by 100k (making up numbers) what good does introducing an ammy that increases rogues overall dps by 80k and rangers by 70k (just making those numbers up while taking the delays of the ranger running forward and back again) do in shortening the gap?)
All in all, i feel like rangers do need a buff, and not in the form of a 2m ammy, rather a direct buff to their skills.