"Be difficult to do"和"doing"在语法上都是正确的，但是在不同的上下文中使用的情况可能会不同。
如果你想表达某个事情具有一般性质，即通常情况下难以完成，那么可以使用"be difficult to do"的结构，例如：
- Learning a new language can be difficult to do.
- Sticking to a diet can be difficult to do.
如果你想表达某个正在进行的动作或事情具有困难性，那么可以使用"doing"的形式，例如：
- I'm finding this math problem quite difficult.
- She's been struggling with her new job, as it's quite difficult.
需要注意的是，在口语中，往往会省略"is"或"are"，直接使用"difficult"和"difficulties"来表达。
be difficult to do还是doing
It is difficult to do something.