At 10m the only thing you can reliably detect is large scale deployments. If you want to do better than that you need paid data which is prohibitively expensive at large scales.
At least in Belgium there’s also a lot of unregistered solar panels to the point that the energy regulator is using satellite imagery to catch and find them…
https://www.brussels.be/how-report-solar-panels-brussels
I have no idea how serious these reasons actually are. It seems registration is free, though you do need to have a digital meter, which may incur extra costs. The system is apparently designed to encourage self-usage rather than feeding back to the grid.
I don't really know the benefit of not registering. To register you may need an inspection, which could be a pain and cost some money. But if you've spent a bunch of money on a system... why not do the seemingly small extra steps?