Druchii.net

The rulebook on page 72 says a character with MR that joins a unit gives it to the whole unit.

The Dark elf book says the cauldron of blood joins and leaves units as if it were a character but otherwise is not treated as a character and can only join and leave units like one.

So doesn't that mean its therefore just a chariot that move in and out of units like a character but isn't a character so doesn't grant MR1 to the whole unit? Because that's an ability beyond just joining and leaving units?

The writing couldn't be more ambiguous if they tried.

The cauldron itself isn't a character, so it can't be challenged etc. However, it joins the unit like a character, and so I would assume that all rules for joining a unit, such as granting magic resistance to the unit, counts.
I'm not the expert but as far as I have observed, it has been consistently ruled that the Cauldron, and the Bloodwrack Shrine give their MR to the unit.