When it gets to the point that the (majority) of competetors are using custom actions, then it stands to reason that more matches will be won by shooters using them. That's not to imply that they don't make for a better gun, especially in some types of competition.
But until about 5 years ago, the 1000 yd heavy gun benchrest record was held for 14 years by John Voinaida , a machine shop teacher who built his own gun on a Rem 721 action. It also had a plain jane 24x Leupold scope.
We would be very very wealthy if we had a dollar for each round sent downrange chasing that record at the Williamsport range alone with all types of guns built by the best of gunsmiths.
Someday the lottery might be won there again, and won for the same reason.