I'm blind and I tried Android 5.0 and it seemed not very good using talkback. Does someone know if there are many differences with the new version?
You should think of "Talkback" as its own app, like any other app in Android. So the version of the Talkback-app usually has not much to do with the Android version. You can have a new version of Android, but an old version of Talkback and vice versa.
Most of the time the Talkback-version in a newer Android is also newer, but you could manually upgrade Talkback.
Some manufacturers like Samsung or Huawai implement their own "TTS - Text to speech engine", which basically is the Talkback-App. For example: The way the TTS reads numbers or weekdays might be quite wrong, the focus-frame might be blue instead of Googles green frame, etc.
From my experience the not-Google TTS-engines are worse than the one from Google, but in the Talkback-Settings you can always switch to the standard Google TTS.
But to answer your question about Android 5 and 7: Sadly I could not find any change logs that exactly describe what has changed. The best link is this: https://nfb.org/blog/vonb-blog/android-60-and-talkback-45-accessibility-improvements
From my experience I can say that the accessibility usually improves from version to version, especially on WebViews and how things are put into context ("5:00" could be read as "5 - pause - zero zero" or as "5 o'clock"). So if you can use a newer version of Android / Talkback, you will benefit from it.