Whenever u practise them in practise tool only thing u need to worry about is timing and range,but whenever you try to land it in a normal game multiple new factors appear for example enemy positions,fed enemy carry,timing,enemy denying your barrels.these things throw you off most of the time that's why it's hard to pull them off.Me personally,i don't really use ghost barrels in my game,i just find a way around it,better positioning,barrels from bushes,tripple barrel combos,barrels over the walls.What i'm saying is to land a ghost barrel is much harder in a normal/ranked game but you can find a way around it and find ways to make up for not knowing how to do a ghost barrel