I hear this all the time, but I question how true it is. I've been running my own mail servers for decades, and I've never had any problems with sending or receiving mail. I suspect anyone who properly configures their server will be fine.
At work I ran email servers professionally and with good deliverability for years. My own email server was arguably longer lived than those at work, just much lower volumes. IP block was clean, DKIM, SPF, rDNS, etc. all setup correctly.
I thought I had no deliverability issues. I interacted with mailing lists regularly, the odd email to friends and family and I was firmly in your camp until I had to deal with a death in the family.
I think this was shortly after Microsoft BPOS became Office365. It became very very clear very very rapidly that to certain orgs I just wasn't hitting the inbox. And there was jack shit I could do about it. That was the end of my mail server, and it's certainly got worse over time.
If I send an email to a corporation’s customer support, or to a distant relation, or to an open source mailing list, and I don’t get a reply, it could be a delivery problem - or it could just be that they didn’t decide to reply.