I think the tricky thing can be that there isn't a good way, to my knowledge, for submitters to check on the status of the submission (beyond emailing the journal directly). That can create issues if the journal sends a response to the submission and it goes to the submitter's spam, for example. Whereas with Submittable, the submitter can go in and check the status of the submission at any time on the platform itself -- regardless of whether or not they actually received the email notification for the response.