it's possible that either the bumpers alone are made of rubber, or the entire buggies are.

Good point. In that light we note that rubber baby-buggy bumpers is ambiguous unless you assume that, if it were only the bumpers that were rubber, then double hyphenization would be required, giving us rubber baby-buggy-bumpers.

Personally, I think that hyphenization is the signpost to the slippery-slope of compound word making, as in the 20th century slide from base ball through the unstable transitional form base-ball to baseball.