What is the difference between a Haifan Baha'i and a Unitarian Baha'i?

If you ask a Haifan Baha'i why they believe the UHJ is infallible, it will not happen that they pull out a single verse or paragraph by Baha'u'llah and say "this is why". No, instead they will say "Let me tell you about The Covenant". They believe the UHJ is infallible not because of some concrete writing by Baha'u'llah, but instead because of the abstraction of "The Covenant". They say that there is a "very clear" concept of "The Covenant" in the Baha'i writings, which is abstracted from several different writings and which if you put them all together point to the UHJ in Haifa as being the infallible authority of all Baha'is in the world. And it is "so clear" that if you deny it then it can only be because you are evil and hate God and are intentionally trying to undermine his religion. This is what Haifan Baha'is believe.

My question is: If "The Covenant" is such an important concept to Baha'u'llah, then why does it only exist in abstract form? Why didn't Baha'u'llah just write a single verse making clear that the UHJ is infallible? Why do you need to show a bunch of bits and pieces of Baha'u'llah's writings from different tablets together, supplemented by additional "context" (e.g. "Muhammad Ali was warned!"), in order to convince people of this alleged "Covenant" concept?

Unitarian Baha'is don't rely on any kind of complex abstraction in order to justify their position. If someone asks a Unitarian Baha'i why they believe Muhammad Ali succeeded Abdul Baha, they just show the verse "Verily, God hath ordained the station of the Greater Branch after the station of the former. Verily, He is the Ordainer, the Wise. We have surely chosen the Greater after the Greatest as a Command from the All-Knowing, the Omniscient!", which is ironically from Baha'u'llah's "Book of my Covenant".

https://www.reddit.com/r/FreeSpeechBahai/comments/131fx4c/the_abstraction_of_the_covenant/