Na nach is nonsensical, but relatively harmless. It’s just saying his name while making pauses.
Yechi is a declaration of faith in the Messiahhood of a deceased rabbi, that he is alive forever, and according to some he is their creator.
The two couldn’t be any more different.
For the record, mainstream breslov looks at na nach as a cute thing that BTs with long hair and guitars are drawn to instead of eastern avodah zara chants. It’s harmless, both in meaning and intent