Thanks for the clarification Justine.
The ab roller is essentially a curl up or crunch. It is safe for most exercisers, but there are some conditions that should be a reason to avoid the exercise or modify the ROM. Things like diastasis recti, spondylolysis, spondylolisthesis, pregnancy, etc.
There is a small number of fitness pros and enthusiasts that are turning against the curl up from research information that cast a negative light on the possible wear and tear to the spine. But the studies that were negative are not totally without flaws. First the studies used the spines from pig corpses and flexed the spines through the complete possible ROM thousands of times till damage occurred. No one (or poor little piggie) should ever take their spine through a full ROM repeatedly in that manner (especially if they removed it and planned to put it back). There were a number of other flaws such as the tissue was no longer viable and unable to repair itself if the piggie had done a reasonable number of flexions and then recovered before doing more flexions, etc. People sometimes just read the headline or listen to the sound bite and make decisions based on not only inaccurate information, but incomplete information.
The ab curl/crunch is probably on of the most widely performed exercises of all time. A vast majority of those suffering from low back pain/injury report improvement in symptoms when using this exercise correctly. The ab roller should not cause problems if the curl up is performed with proper form and ROM.