If you are playing better backs, true, they can be harder to tackle. That is why you work on tackling every day. For bigger backs, you work on hitting low. It is one thing to "teach" good technique; another thing to work on it every day so you continue to improve. If you want to be a good tackling team, you work on tackling. If you want to be a good passing team, you work on throwing the ball, etc. IMO - too many teams teach tackling early, then never really spend much time on it during the season because of everything else going on. Many coaches are concerned about getting kids hurt in practice. But there are ways to work tackling and minimize the risk of injury at the same time.