The issue with FDE in Android has for long been the lack of combining strong passwords with a pattern lock or pin lock for unlocking the screen. In other words, your encryption key is only as strong as the pin code or password you are willing to put in every time you open your screen lock. Who wants to type in a 20+ password every time they open their screen lock?
Are you sure?
For my Android phone I activated FDE. On boot I have to enter the FDE password, which is independent from the lock screen password/pattern/face unlock.
So on boot I enter the complex password once, and later I use the less complex pattern to unlock my running phone.
My phone is Running Android 4.4.4 (Cyanogen CM11S).