I have come across posts where contributors (in the comments) state that you cannot use a particular feature from Unearthed Arcana (UA) because the player had already chosen to multi-class.
Where does this notion come from: is there a citation from Jeremy Crawford or Sage Advice, or a quote from UA articles stating that UA is not designed with multiclassing in mind?
I realise the DM has the final say on this, but is there an official statement in the published materials or a tweet to support this, or is it merely based on opinion?
The main argument I've seen is this: It is UA material and not part of the main set of rules yet; it has not been adjusted for multi-classing. Therefore players who multi-class are not allowed to use materials from UA.
I am looking for reliable sources to back up this argument.