So firstly, I would say what I've been working for me.
1)Which leagues do you find the most predictable in terms of match outcomes?
In soccer, I believe the English Premier and La Liga can be more predictable for me because I watched them the most. When you watch a league back to back, you will know teams that are better to bet on. So it doesn't matter the league, what matters is what league you watch the most.
2)And which leagues are the most unpredictable, where you can’t always rely on teams' form or stats?
Unpopular leagues are the most unpredictable. The reason is because I am not familiar with those teams.