At what difference in team strength does it not matter and it comes down more to tactical choices? Obviously if I have a +30 strength difference to my opponent I'm gonna roll. But if I only have say a +10 difference do I stand a chance of losing?

There is a part of randomness in the game, so the big club not always win.

Be careful with team overall rating. If both opponents play "normal" obviously you are way better with +30. However, if your opponent play with a style for his team, it's low overall rating can hide a huge boost in stats when team are on the pitch.