Google did not like external storage accessories such as SD cards. You can’t install applications on external microSD card. However, things changed after Android 6.0 Marshmallow. Yup, Android 6.0 introduces the ability to adopt external storage media to act like internal storage. Applications and data can be stored at external storage from now on. External storage media is formatted and encrypted when adopted. And can work with a single Android device at a time only. So it is safe to store both apps and private data for all users.
Adoptable storage (or flex storage) uses GPT and has file storage limit of ~9ZB. Yup, you can attach virtually any size external storage to it. As you can see from above screenshot, I managed to attach 2TB HDD to my Android 7.0 Nougat HiMedia Q10 Pro TV Box. However, it is limited to external storage like SD cards and not USB devices for phone and tablet because they can be removed easily to avoid accidental data loss or corruption. TV-style device like Android TV Box can do so with external USB drive. That’s one of the exceptions.
External storage like microSD card has slower read/write performance than internal storage. Therefore, system will run benchmark and compares its performance against internal storage. If the adopted device is significantly slower than internal storage, it will warn that it might have possibly degraded experience. Furthermore, encryption makes it even slower then. In addition, you can’t access (copy or paste) file into adoptable storage manually through file explorer (unless your device has root access). Do take note on that.
Should you enable adoptable storage on your Android device? Yes, if your devices have less internal storage like 8GB only. It will be filled up easily with few large 3D games. And adoptable storage is the only way that let you install more applications and games on external storage. For your information, all new applications and games will be installed on adoptable storage automatically. But you can always move them between internal and external storage any time you want.