I'm experiencing this bug for a few years now. Back then with a Radeon HD 7770, a R9 380 and now a RX 480, all with free drivers (radeon/AMDGPU) on different motherboards. It's very simple: Sometimes, not every time, but by around 50% of tries, the screen freezes when I initiate the "Desktop Grid"/"Raster" effect. Sometimes this doesn't freeze the screen directly but seems to make the WM somehow 'sick' so it crashes/freezes at a random other effect over the next half hour. I can only reset the X server, lose work in the process and do a relogin.
All other effects run fine without crashing Kwin or freezing the screen. I thought a long time this must be a somehow driver related bug, pointing the finger at AMD's poor driver quality in the past, but after three years, this is rather unlikely.