While this update seems to have caused issues for over a third of users according to the poll, I understand that switching to GTK had to happen at some point. Perhaps it's better that these issues occur now rather than later, before the major release of 16.6.
While my system has never broken due to a Manjaro update cycle, there was one update cycle last year that made it behave a bit quirky, however on the next update cycle afterwards, things were smoothed out again and worked perfectly again.
I suspect this time will be similar, however perhaps it might take a few update cycles before everything is perfect again, because quite a few packages and themes have to adapt to the new GTK.