It's not just overload, but also fault clearning. If you have a 100A panel that's heavily loaded, and suffer a high-current fault on a 40A or 50A branch circuit breaker that sees only itermittant load, it's going to trip the main first almost every time. The proper solution would be to adjust the trip curves so that a branch circuit will always clear the fault prior to the main opening, but if the main isn't adjustible... there's nothing you can do, you're stuck with whatever it's set at. If it's set at the same delay as the branch circuits, you're SOL for any fault that exceeds that threshold.
With fuses, you could at least select a slower fuse, but proper coordination is simply not possible with small panels like it is with larger adjustible trip switchboards. It's actually quite a big problem in certain segments that require coordination of these small panels.
