To point out that Hebrews is a covenant document is to say something true and important. But it is a dangerous thing to point out. Because it is possible to communicate by those words the gross falsehood that Hebrews is a special subchristian kind of document. Romans 8 is the pure truth about believers and their absolute assurance of salvation, but Hebrews only contains the warnings it does because it is a “covenant doecument.”
But the truth is that Romans is a covenant document too. And so is the Bible.