Clearly it's debatable as to whether it is acceptable in American Football given some of the comments we've seen on here from North Americans but it doesn't have the issue of disrupting the game that proper football would have given the stop/start nature. Same with cricket, where it does work extremely well in my opinion.
I suppose this is one of the two threads where we can risk going all-smhinto and repeating ourselves .... Brexit being the other, although on that one I get so confused that I'm not sure who stands for what!
I have posted on "instant replay" in North American sports before, but for NFL specifically ......
1) when it works, it works - this past weekend was the first round of playoffs and it came into use a number of times at critical stages.
2) it doesn't fix an ambiguous rule. Earlier on this tread, hand-ball was mentioned as something that is unclear by nature. Same in the NFL. They have struggled with a "legal catch" definition for a few years, is still a problem. So even if you see it a number of times on replay, it is still subject to interpretation.
3) And not everything is reviewable, which can be very disconcerting when an incorrect call is allowed to stand because it is not, by the laws of the game, subject to review.
4) agree that NFL is stop/start, but it is also a game of momentum. A team on offense might have the opposition on the run, defense tired. The QB hurries the team up to the line, but then the whistle gets blown for a replay look.
5) NFL games run at least 3 hours already ... replay does add a few minutes to that.
6) there is an element of anti-climax when replay is required, especially on a touchdown - did ball break the plane of the goal line or not? As someone said, can be a big let-down or lead to a double celebration.
7) NFL teams can call three time-outs per half (wouldn't that be something for Eddie to have in his back pocket when we are having one of our defensive "spells"). A coach's challenge, if shown to be wrong, costs one of those time-outs.
8) Officials are
slowly figuring out that the play should be allowed to run - only blow the whistle when certain the play is dead. This allows replay to correct errors (replay is looked at in the background for all scoring plays and turnovers ... i.e. no coach's challenge required). Linos should definitely take this approach when VAR comes in.
9) NFL has 7 on field officials. On questionable plays - in/out of bounds being a good one, penalty calls being another - the officials do huddle to try to get it right.
All in all, as I said at the top, when it works, it works. But will not fix all ills.