I sometimes go for the ugly but strong assertion:
    `assertEquals(ImmutableSet.copyOf(bucketA.getMembers()), ImmutableSet.of(child1, child2), "members="+bucketA.getMembers());`

The string at the end is particularly annoying, but otherwise it just says the sizes didn't match.
Maybe it's time we added a utility for `Asserts.assertContentsEqual(Iterable<?>, Iterable<?>)`