In every sport it's taken a long time to bed the system in, with trial and error, tweaks here and there to get it right. Problem is that by definition they have to really do it in high profile matches where all the technology is there, so it creates a load of attention and cynicism as something doesn't go smoothly (and doesn't help that most pundits are thick as shit). I also see a problem where referees end up becoming overly cautious and check more than is strictly necesssary. If you compare it to run outs in cricket, it got to a point where almost every single decision was sent upstairs even if they're out by a mile. The good thing about what they finally implemented in cricket is they put it in the hands of the players so it's a bit of a 'put up, or shut up' situation, if you think the decision is wrong you appeal it, if the ref was right you lose your appeal. I'd prefer that kind of thing in football, partly because I think it would improve player behaviour if there was less incentive to con the ref and costing your team appeals.