If every house has an ADU, then the effective number of housing units in that area has been doubled, putting downwards pressure on housing prices.
If people bought their houses anticipating high rental income to make ends meet, that's a separate problem, and less bad than "people are becoming homeless because there are too few housing units".
If people bought their houses anticipating high rental income to make ends meet, that's a separate problem, and less bad than "people are becoming homeless because there are too few housing units".