This is not (always) true.

Maddy comes with a lot of the 'good stuff' like DKIM and DMARC ready to be used, whereas setting that up yourself with Postfix was a pain (from my experience a few years ago) and probably meant you didn't bother.

I've had better deliverability, especially to Gmail and Microsoft, since using Maddy, probably for this reason.

Now, of course, I must admit that self-hosted e-mail seems like it will always inevitably run into a blocklist problem, usually because of IP neighbours. But personally, I've managed to avoid that in the last year that I've been running Maddy -- main exception is I had to fill in Microsoft's silly form to get unblocked at the start (but I've had to do this for every deployment I've ever done).