The Intermission

This sapper does not damage enemy buildings and cannot be broken by enemy engies/pyros. It auto-destroys itself after 8 seconds on placement.

-100% Sapper Damage Penalty
+100% Damage Resistance
Destroys itself after 8 seconds
Can only be placed on one building before cooldown
User becomes undisguised on use

If these stats made it in, it would be very useful during uber pushes. Could also make atleast a slight difference if sapping a teleporter if the spy in question is struggling to help their team.
However it would not be consistent against multiple engineer buildings, sentry nests and all that.