I'd say 8/10 is maybe a bit optimistic (at least if you remove the blatant/'obvious' ones where the forward knows he is off), in realtime when there is a close call I'd say it's a bit lower. I do get surprised watching the replays sometimes.
There's an argument to suggest that even 8/10 isn't good enough, if we have the potential to do better.
Also I don't think it is quite that easy for the linesman, they can't ALWAYS be looking exactly straight down the line (wrongfooted by an agile defender, say, or the crazy situation you get sometimes where defenders are moving in different directions so you have to switch which is the last man), and that isn't their only job, they also need to know the precise moment the ball was played, we may be difficult if they need to be looking 40 yards back up the pitch for a long ball punted forward, meanwhile they've got the centrehalf pushing up and the striker starting his run the other way.
Yeah I was probably being a bit harsh with the 8/10 call, I still had a small amount of frustration in me from last Saturday.
They will only be using it for goal line calls, they won't use anything which causes the game to be stopped e.g. challenge system, hawk-eye etc.