It will depend a lot on the area. If it is a decent area and the bike cannot be seen from the street it is pretty safe in the backyard, specially if the gates are actually locked. You can always add a cable lock around the wheels and even lock that to something. Cover also helps for anyone just looking over the fence scoping it out. Maybe a motion light.
Unless it is very specifically targeted thieves want quick and easy. They can defeat pretty much everything but they also don't want to spend a bunch of time fiddling around increasing the risk of being caught. Make it a pain in the ass.
Now if you are in a bad area, that may be different as people are scoping things out all the time. Growing up (Falls) I had my bicycle stolen from my back yard, it was there maybe 30 minutes unlocked out of sight. Stuff got stolen all the time, BBQs, etc. had to be locked down. Where I live now I am always shocked that kids just leave their bicycles in their front yards. At my kids school, some kids even leave their bicycles unlocked or just have one of those supper skinny cables locks, crazy.
The other wild card these days are drones. Friend caught "kids" actually teenagers with a camera equipped drone scoping backyards for pot plants.... no need to fence hop.
Not bike related, but be careful if your garage is attached and there is a door into the house. Thieves break into the garage (methods as discussed in this thread), then the house (all when they know no one is home). Take their time picking through the house, load the best loot up in the garage, pull up a cube van load up in minutes and split.