麻烦给个谚语：任何事情都有两面性.

这些都是我写作的时候常用的．觉得不饶口也好记．
every coin has its two sides.
everything has its pros and cons.(这句比较口语一点)
...is a two-edged sword.
...是把双刃剑．
No garden without its weeds.
没有无杂草的花园（有利有弊）
everything has double folds.
every medal has its reverse.
补充问题的回答:
推荐用这句:
It is essential to examine every angle in order to fully understand the nature of a perplexing problem.
为了全面了解一个复杂问题的本质,很有必要从各个方面对事件进行调查．