I honestly don't think the captain's challenge is a thing that can be effective in Rugby League.
You look at other sports that have implemented this type of thing, none of those sports are momentum based where if one team gets on a roll and has the other team puffing that having a 2-3 minute delay for a decision review completely negates the flow of the game, in Rugby League it most certainly does.
It's just mind boggling to me, they did all these things to speed up the game then they implement an open season challenge system which teams have started to use tactically to arrest momentum in a game.
I think what they've done in Football (Soccer), is perfect. The video referees are actively reviewing decisions whilst play continues, big decisions I mean, penalties and such. Sports like Cricket and Tennis have had no issues implementing review systems because there's a stoppage after every play, it doesn't take anything away from the sport.
I agree fully with the concept being based around momentum. It is what makes this sport so amazing, but it is also its Achilles' Heel.
In a perfect world the Ref's decisions should balance out if it is free flowing and mostly innocuous minor infringements. Obviously, the better refs would ping momentum-altering plays, both ways, e.g., slowing the ruck, and moving forward off the mark, etc.
The MAJOR issue facing this momentum-based sport is the 6-again system. It grants far too much momentum and can easily be manipulated. Furthermore, it is compounding, i.e., once you have some, you're likely to continue to get more, particularly early in the game.
Secondary to this are any large momentum-changing calls such as the 20m, 7-tackle restarts from a poor kick into the in-goal area. Such a minor error doesn't deserve such a large momentum swing against. Fair enough if you kick from 40+m out and the ball goes dead in goal, you're not building pressure, but if you're close to scoring, put in a short kick, or knock-on in goal, why such a huge momentum swing? That is completely stupid in the context of the game.
However, the NRL is not a smart sporting body.