In my limited experience of reading about failures, rather than direclty seeing them, most of the racks that fail that are rated for (sometimes much) more than the load they carry fail in bumpy situations, but last forever in flat road situations.
My expereinces are from my own homemade racks, and a couple of crappy used ones, and various types of steel basket racks, carrying various cargo rather than batteries, mostly.
The thing you have to remember is that there is inertia and leverage and sway when figuring actual weight capacity with a rack,
I don't know what the math is for it, but weight that's out at the back end of a rack vs near the front will be multiplied during both bump and after the fall back to the ground. Most likely, in many bumpy situations, that's going to exceed the rack's specs each big bump you hit. Eventually, it's going to cause a final failure, after many stresses to it.
Not only is it vertical bumps, but also the sideways sway of the rack both as you pedal and as you lean into turns, etc. Tiny stresses that it seems few racks are designed to deal with just add up over time.
A rack that is supported by posts at the dropouts as well as seatpost is going to be stronger than a single seatpost-mount-point type, but even these will fail. One issue is that typically the dropout supports are on pivots so that it will fit many different bikes. That means there is no solid mounting there, so sidesway is even worse than if it were all welded together. Often the front rack support on those that *are* welded is thin and can't prevent sway at the front, or allows the rack to bow up and down at the front end, which stresses the welds on the vertical dropout supports in a different direction (as well as the sway problem). Eventually, the little cracks join up and become big cracks, spread further, and stuff breaks off.
Any rack that only has one supported end is going to be even less likely to survive, as it can't do anything at all about either vertical or horizontal movement--it's just like you holding your arm out and jumping up and down and leaning back and forth. Your hand will end up swaying all over the place. It's got joints designed to accomodate that, so it doesnt' usually break and fall off, but the rack isn't meant to do that so it will eventually come apart, leaving your battery on the road behind you somewhere.
Basically, in bumpy sitautions, all racks will rpobably fail eventually. The more solidly built a rack is, with proper triangulation against all axes of movement, the more likely it will outlast your need of it. Most racks have no triangulation at all, and can sway and move in all axes, so their joints break from movement they're not intended to handle.