I think they are probably fairly accurate. There’s always room for individuals or congregation secretaries to falsify reports to some extent, but I doubt it’s happening to any significant degree.
Memorial attendance is a good indicator, although memorial attendance figures during the pandemic were all over the place, probably due to a combination of people reacting during a crisis, and the fact that Zoom complicated what counts as attendance.
One figure that is near impossible to falsify is the number of congregations, so I pay particlar attention to those numbers. The increase in congregations is much slower than publishers in Ireland. Yet, compared with other churches in Europe, any increase in number of congregations is an achievement against a backdrop of churches in steep decline and secularisation.