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.
Friendly matches are being played a lot these days as teams that will play in preliminary qualification for the Champions League want to see the condition of their players,of course in such games it is very difficult to predict the outcome of the game but not only that,also it is very difficult to predict with goals,both teams to score or other type of bets and the reason is simple,we don't know how the players of each team will behave as they have no real objectives in these games except to test their form.Therefore it is much better to refrain from betting on such games or the risk of losing money will be bigger than in normal games that have an objective in them.