Woodworking instruments (including disks that can be mounted on an angle grinder) generally have teeth that in some circumstances can "grab" a piece of wood and fail to cut it trough. Wood has variable and anisotropic properties (and in some cases it also contains long-forgotten nails) that promote such events.
In this case, the instrument with its motor and mainly with its inertia exerts a great deal of force and torque on its support.
When the support is steel or cast iron bolted to a heavy table or to the floor - good. The worst that could happen is that the instrument breaks or some wood piece could fly at considerable speed across the workshop.
When the support is the carpenter's hands, the instrument itself gets wild.
Lower RPM does not in itself make things safer.
At lower RPM the instrument is more likely to fail to cut some harder part and kick back, at the same time the kickback is not much safer.
What can be considered safe, then?
The only thing I personally do with an angle grinder on wood is to cut something small and well-fixed using a disk for metalwork. It has no teeth so it can't easily "grab" something and it also makes the whole process quite inefficient. The disk actually burns its way in the wood.