Properly rated, properly used, ratchet straps will hold a spare in the rig. Nothing can be 100% but this is damn close. Have you ever looked at off road racers? They often have impact loads measured in the 100G range, often use a ratchet strap to hold the spare and have never seen one come loose. Lots of jumps, accidents, roll overs, etc, never seen one come loose, ever.
Ratchet straps are commonly used to hold very heavy loads on semi trailers, how many do you see falling off? Do you allow your kids to ride in your car around/following that type of foolishness? One would think that a load of pipe, tens of thousands of pounds, falling off at highway speed would be at least as bad as a spare?
A good quality strap, properly fastened, will easily hold a 100lb spare. Yes, it will wiggle a bit, it's a rubber tire, held by a nylon strap, so has slight give, but isn't significantly going anywhere. If involved in an accident with high enough loads the break it, the spare is going to be the least of the worries, likely the occupants will never worry again, regardless of what the spare did.