It is working for some people 
.
There can be no 100% reliable method, because there is no 100% reliability in the way most software is made. This also applies to TWRP (just feed your favourite internet search engine with “createtarfork() process ended with error 255”.)
So the real question is … what’s your Plan B if TWRP fails?
You got the Internal Storage covered as long as you confirm it got successfully copied to your computer or wherever.
You can install the OS from scratch (better download and keep the OS install files matching your backups).
Apart from the /data/media folder (= Internal Storage) that leaves the rest of the data partition. This holds e.g. your contacts, your SMS/MMS messages, your call history, data saved there by Apps.
There are tools like MyPhoneExplorer to sync contacts, messages and call history to a computer.
Google is constantly tightening some screws, so it isn’t as convenient to restore messages as it was some time ago, and I don’t know whether in recent Android the call history can be restored at all to a phone, but at least this data is not lost.
Apps are covered by their availability in Stores, of course.
If you want to quicken the install or if you need a particular version of an App for some feature or compatibility issue, you could extract the respective App APKs (with e.g. SAI, since there are split APKs around which standard APK extractors don’t handle correctly).
App data in the data partition but not in Internal Storage is a problem.
You would have to rely on sync/backup/export functionality of every single App which you consider important. Well … let’s hope TWRP works .