In this technical article it is stated that changing the nullable property in a field from true to false or vice versa can be problematic. However, on ArcGIS Online I find it even more problematic that the only way to edit the nullable property in a field is to publish the service or overwrite the existing hosted service. For example, overwriting an existing hosted feature layer is in my experience not possible when using a hosted layer with different hosted layer views. And I encountered situations with multiple webmaps and webapps using one hosted feature layer where the stated workaround means loss of productive hours (building, designing and testing). Essentially it means starting over from the desktop in situations where it is not possible to simply overwrite the existing service. This is tedious.
The data risk is not to be underestimated but a solution for this issue may save the day. After all: fiddling with settings on the REST page is not for everyone. Also I think that a proper warning can be enough to increase the data risk even further.