be difficult for sb. 结构，除非强调for sb.可以提前。

对于这个比较句而言，如果 for sb. 提前至句首，主句可以处理，但 as 从句不好处理。

这原本就是一个as ...as... 比较主从句。不知网友为何要改变结构呢？

演绎一下：

1. This must have been difficult for them.

划线部分不要拆开，在形容词前加程度副词as：

This must have been as difficult for them.

2. 既然比较，需要有比较的从句照应：

This must have been as difficult for them as landing on the moon is for us today.

3. 这样，as 从句中，与主句相同的比较项 difficult 空缺（gap 在 is 后）。结构是完全对应的。