The number of axles has nothing to do with how easy it is to back a trailer. The factor that matters is the length of the trailer and the distance between the trailer axle and the hitch. I have a short utility trailer that is very difficult to back because the "turn time" is very short. I also have a trailer built to carry my 18' sea kayak that is extremely easy to back. Both of those trailers are single axle. The longer the trailer, the easier it is to plan your backup.
A single axle trailer will not tip over if you have a flat unless you are on a very sharply crowned road. The tire is only about 4" to 6" tall so the trailer does not tip very much. It is the same as driving up on 4 to 6 leveling blocks on a level road.