Well of course it is.
Having to watch the ball, and players who are often moving at speed in differing directions all at the same time means its impossible for linesman to actually judge a lot of decisions accuratly.
Thus they guess but that doesn't stop people whinging and calling them crap after they've watched numerous slow mo replays and the linesman guessed wrong.
Aye, I think the whole profession is founded on lies. There's no such thing as a bad linesman, it's just a cabal of brazen chancers claiming they can make scientifically impossible calls.