Not all IPs are equal. I run two separate email servers which are almost identically set up. One for my business and one for my personal use, both with negligible outgoing email traffic (less than 50 emails a day). My business server is with a quality ISP who cares about their network and deliverability is fine. The other, for my personal use, is on a cheapo VPS provider and the IP gets blacklisted enough because of neighboring IPs in the /24 sending spam that I now relay all of my emails through Sendgrid in order to get any kind of reasonable deliverability.

I think this is good advice. The overhead in operations to host and maintain a custom setup is non-negligible, but at least you might get better delivery rates. One of the annoying things about the commercial relays is that the reported delivery rates can't be trusted as they are based on the receiving server actually reporting accurate responses. In lots of cases the receiving server, if suspecting a spam email, will just report that they received the email and then pass it straight into the proverbial shredder.

So while a relay might say you have 99% delivery rate, the actual number might be closer to 90% and you would never know unless your customers complain about missing emails.