Whilst mental illness of itself would not warrant a disc offence you have to look at the impact of a persons symptomology on their JW life. Think about the pressure to attend every meeting, to face public in field circus when you are barely able to leave the house. You can't just tumble out of bed to a meet you are expected to be suited booted bright eyed and bushy tailed fully participating in every study. Absolutely no allowances are made short of death. If you don't measurement up you could face a potential disciplinary. Also MH is a wild and woolly world especially psychosis. Eternal themes abound, lots of believing to hear from/ be God which will start the good old demonic meddling rumour mill.
So what I'm saying I guess is that the rules of the Jdub club indirectly preclude those with Mental illness so chances are you wouldn't join if you did you wouldn't last long. In a similar vein suicide rate chances are higher rates are among those who have left or want too. The pressure coupled with lack of protective factors would increase the incidence for those in it and happy probably not an issue