It's annoying when I use W on an incoming minion wave but it bounces only first 4 minions because I don't see the 5th one yet. It makes no sense. It's not like she's controlling her boomerang after she threw it.
A similar example would be 
E. It still bounces when he doesn't have vision of a target.
I agree Riot needs to increase the ad ratios of Sivirs W
Because unlike other bounce skills, it has infinit bounces, so her harras and zoneing potential would be absurd if the bounces weren't limited by vision. Just walk up to an incomeing minion wave and hit the first, and any opponent in ~400range of any minion would eat damage without even seeing sivir, so with little warning. An come to lategame, a single bounce already hurts alot.
Again, it's mostly the infinit bounces part. If you throw fiddles E on a minion wave, you might get 1 hit on a champ in the dark, but most of the Es damage will hit minions, so throwing it into the minions means you use alot mana and put one of your main damage sources on decent CD to eventuall hit someone with 1/6 of his spell. Sivir would just W with little manacost or cd into the minionwave and gain full effect of her bounces on the minions and anyone near them.
I feel like you could easily argue that she's throwing the boomerang so that it specifically bounces off those targets, as opposed to the fiddles crow which just attacks what it sees
Um... I guess it's because crows have eyes and crazy cross-boomerang things don't? I have no idea.
Probably to keep it from bouncing to enemies hiding in bushes.
There are some differences between them
Siv's ability is -an auto modifier -affects three autos -allows infinite bounces, but can't bounce to the same target multiple times
Fiddle's ability is -an actual spell with cast time, etc. -has limited bounces but can bounce to the same target multiple times
My thoughts are that Fiddle's crow is limited, and rather than allowing it to fizzle and cut off his damage output, it is allowed to continue to seek targets that are not visible to Fiddle. This means in the fog of war and in the bush (but I'm pretty sure they removed it bouncing to invisible units - might be wrong).
Siv, on the other hand, can get her rangs to bounce from bot to top lane given the right vision and target lineups (yes, that is a neigh-impossible situation, but roll with it). Because it has no limitation on number of bounces, to prevent it from going crazy they made it only bounce to things that are visible.
So yea, they seem very similar on the surface, but I think they are significantly different in purpose, especially when viewed through the lens of what the characters are trying to do in the game, as they play very differently.
Personal opinion - Fiddles crow is complete bull and shouldn't find you in bushes
Same with Jhin Q
Fiddlesticks at least has an excuse. He sucks in most situations, so I'm okay with him getting some bullshit.
I would agree to this because sivir W works like a fiddle E, and it is one of the main trading abilities with sivir, I suggest change this to the same technique as the fiddle E.
I've seen it hit me in unwarded botlane bushes as an enemy support, and just like fiddle's E it has a max bounce range. Though fiddle's an insane amount longer/buggy-er Sivir's W will bounce to unseen enemies in bushes but it just doesn't bounce insanely far. I can attest if you try in a few practice games it will bounce to hidden enemies but length-wise it's not the longest bounce in-game so that's maybe what you're dealing with.
