The subject line says it all really. Mentioned by Sune Vuorela on IRC so I'm recording it here so I can avoid forgetting it.

Suggested command line flag: --list-all-failed-builds

Workaround for now (assuming persistent data kept in ~/.kdesrc-build-data):

$ perl -0777 -E 'my $vars = eval <>; while(my ($k, $v) = each (%$vars)) { say "$k" if $v->{"failure-count"} > 0 }' < ~/.kdesrc-build-data

(the -0777 forces the <> operator to read the whole input in at once).