Tunze osmolator has a high level cutoff... they're pretty safe, but yeah... you should still plumb your system for failure. Depending on your setup, you might siphon all the water from the reservoir under the right conditions.
I keep enough space in my sump to completely empty a full ATO reservoir, AND have all the pumps powered off. Back flow valves and such are great, but if you count on them to keep from flooding your living room, well, all mechanical devices fail at some point.
A cheap battery powered water alarm is cheap insurance!