Not just good enough, they fit their niche almost perfectly.

Die often? Large passals that reach maturity relatively quickly. Have to risk ingesting potent toxins? Remember what almost killed you just long enough to get away from wherever it was(roughly 2 weeks). Not built for fighting? Fake your death so convincingly, vultures don't want to be near you. Nutrition might be scarce? Gather your feces, it will see you through.

Hell, even the fact that the Virginia Opossum can't adapt well to extreme heat or cold is a benefit to them, the resources they're driven to(such as actively running water, dense undergrowth for cover and foraging, and trees that can support their weight) can be scarce.