I believe there are many gamblers who like to gamble on International competitions because they know how to maximize the chance from the matches. Some people may think that predicting international matches is easier than domestic league but some other may say the opposite.
The complexity of predicting winners depends on how well the bettor knows about the team composition. I think there is no difference between domestic and international betting difficulty as long as we know the skills of all the players involved.
There is a slight difference, and that is the type of competition. I feel like UCL versus World cup could be a good example, they are closer to each other, but if we are talking about the league, then there is a big difference. When it is a tournament format, there are teams who loses and goes home, but when it is a league format then even City lost games and still became champions, hence losing is guaranteed almost all the time.
So, you just have to suck up and try to bet better, plus there is no ties in tournament format and one has to move to next stage, whereas in the leagues there are tons of tie games. These are very important distinctions.