The problem is small little things you would never think of can happen that can cause failures to occur.
In 2015 I was in AR, 2000-2500 miles into my trip when the rack insert, which holds the rack to the bike fell down into the seat stay. I ended up dragging for a very short distance the kitty liter buckets behind as I already had the other top bolt holding the rack on come out on me, sometime earlier. This happened after dark and I didn't have anything other than an old tube to hold the rack up in place. By the time I got to a hardware store the next day I had lost a third bolt down at the dropout and fortunately about an hour before I lost it I passed by a fully intact bungee cord, about six inches long, laying on the ground and for some stupid reason I went back and picked it up. The bungee cord ended up being my saving grace until I could get the hardware store and replace the bolts.
I got up into MN, around 4500 miles into the trip and found I had spokes pulling through the rim. It would be the first of three back to back wheels I would have the same problem with, premature failure due to spokes pulling through the rim...the most recent wheel, was 3400 unloaded miles before the spokes pulled through...can you say bad frame.
By the time I got into NW Ohio and stopped thankfully, earlier than planned at my mom's house, I had broken all 4 welds on the rack and was barely hanging on with anything. I ended up having to replace the rack before I could ever leave her place.
The craziest things can go wrong, and at the craziest/worst possible times. Somethings you can be prepared for, like you say a flat tire, broken spoke, etc, but something you can't even begin to predict could go wrong, like replacing 3 rear wheels on the same bike in one year...a brand new bike right before I left for the trip.