Flexbox makes your life easy and so you're addicted to it. Using less lines of code, you can do a lot of work. But there can be certain cases where you don't really need that stuff. IMO, if you can do your modification without using flexbox, you shouldn't use it. However, if you think that if you use flexbox, it will be maintainable, you can use flexbox.
Think about it this way: you can do some modification without using jQuery but you know that if you use jQuery, your work will become easier and so you tend to use jQuery.
So, I would suggest you to decide based on test cases. If you think that you can modify without using flexbox, then don't use it. However, if you think that it will take much more time and a lot of hacky ways to get your job done, then use flexbox if the scope actually permits you.
Hope this helps! :)