Friendly match are hard to predict because they are less entertaining and that is because it is just a friendly match, you won't get the same performance that you can get out of a La Liga football match or Champions League since this is just a friendly match you should expect less performance and possible manipulation because when things are not be taken very seriously it will be easier for the football coach to take bribe behind the back and take accept defeat for the money.
I also have some friends that are into football gambling specifically, They always complain about friendly matches and none of them have ever bet their money on any friendly matches because they believe that it is less fun and more harder to predict.
For me, playing friendly matches is similar to training; I don't believe the players will force themselves to play difficult games; when playing friendly matches, you will see small clubs beating big clubs; you can never predict who will win the match; friendly games are similar to making the players more fit before the league begins; and the coach rests some key players due to injury. When a player is injured during a friendly, it is a terrible thing to happen to him because it damages his team and keeps him on the sidelines. Playing friendly is not so essential, and when it comes to predicting, you may bet money on a large club to beat a little club and lose.