@Ragarath Strewth! Fusey main here (except in Arenas).
There are two ways of using the Ult - defensively and offensively.
Defensively you just wang it behind you then run away.
Offensively, you need to have a second or two free to get the placement spot on. Wherever the bottom of the placement guide is touching, that's where it will deploy, so if you're aiming ahead of you but that bit of the guide is on a rock just a couple of yards ahead of you then that's where it is going to deploy.
If you want to place it where there are obstacles in the way, then tough, you can't. it's a design 'feature' - if there's a wall 10 feet ahead of you but beyond that distant mountains then you'll only be able to fire it at the wall, not 10 feet behind the wall (which is probably where you want it to go).
If I am going to use the ult offensively, then I try and get as high as possible so I get as clear a view as possible.
It's not a perfect ult but if you could fire it 100% accurately every time it would be too OP, like Seer's and Gibby's ults combined.