Is it reliable to save some app files in /storage/emulated/0/? I already read that /sdcard/ is a symlink to the actual path of /storage/emulated/0/.
I'm an Android app developer using Apache Cordova, wherein cordova.file.externalRootDirectory is indeed file:///storage/emulated/0/.
Is it always fully reliable to save some app internal files at /storage/emulated/0/path/to/app, considering that the user might not have an external physical memory card on their phone?
Is it always fully reliable to save some app internal files at /storage/emulated/0/path/to/app?

No. External shared storage isn't reliable, as documented under getExternalFilesDir() and other related documentation, because of multiple possible reasons:

User or any other app with full storage access can read, modify or delete the files.
App's private directories in external storage get deleted when the app is uninstalled.
User might be using physically external SD card as the only primary shared external storage. So the data is lost when the card is removed.

Better use internal storage (getFilesDir() and sisters) for data security and reliability. For more details:

Android's Storage Journey
Where Android apps store data?
What is /storage/emulated/0/?