Rules, rules, rules. Rules are what makes the game interesting and fun to play. However, an abundance of rules can slow the game down, make it too difficult to understand, or even make it boring. If you add too many (difficult) rules, this may be why you got this post. A different reason could be that the effects of your content could be too difficult to predict. It's hard to say what would happen if every player gets two bonus actions per turn, therefore we shall not add such content.
When your homebrew gets declined with this reason and you wish to resubmit your content, please avoid rules that are difficult to understand or rules for which the effects are difficult to estimate.