I noticed that passing multiple checkbox values passed only the first value each time. I modifed marketo_ma_webform_webform_submission_insert to run a check for multiple values, and if so, implode. Starting at line 111:

elseif (isset($submission->data[$cid][0])) {
// also need to account for multiple values
if (isset($submission->data[$cid][1])) {
$data[$key] = implode(", ", $submission->data[$cid]);
} else {
$data[$key] = $submission->data[$cid][0];
}
}

If this is worth adding to the next version, I can create a patch. The main issue I see with my code is users may want to use a different separator than a comma (perhaps a pipe?). Thanks!