When the WT says that many who partake are mentally unstable they are using a propaganda technique known as "framing an argument."
They are, in effect, presenting an argument that has only two sides: You either have the WT's view of who should partake, or, you are mentally challenged in some way.
What they NEVER mention is that many of those who now partake are doing so because they believe this is what Jesus wants all Christians to do. From the WT's perspective, such a view strikes at the heart of their authority structure and can never be acknowledged.
IMO, the WT's 'mentally unstable' idea amounts to a deliberate lie which places them outside the very covenant they claim to be part of. (Rev 22:15)