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.
It's true that that is the reality and if we have full expectations if the superior team can win easily against a weak team then that is a big mistake and often the results are reversed if the weak team wins. Maybe that's basically we can judge the results of the friendly match and also this is not an official competition where a good team should win the match.
On the other hand, friendly matches are very good for testing some young players who have just entered the national team squad and it can be a good benchmark for the coach to give his assessment later. For gambling, I think that friendly matches are really not worth the trouble, but as long as you only intend to find entertainment by risking money on the match, then it's okay even if the results are not as expected.