Rivendell Bow is effectively one of two things: 1) a less expensive Dunedain Cache that only attaches to certain characters or 2) for characters like Legolas or Haldir, a non-restricted attack boost. So while Rivendell Bow may not be the strongest weapon on the block, it is certainly a very good one.
Attach to a Noldor or Silvan character, or to Aragorn. Limit 1 per character.
Attached character gains ranged.
If attached character has the printed ranged keyword, it gets +1 during a ranged attack.
The Watcher in the Water #57. Tactics.