And everything works just peachy. Until it doesn't.
The problems are that:
1) the determination of "works well" or "doesn't" is almost entirely out of your hands. A DNSBL or major email service provider decides they don't like the tilt of your kilt and it's titsup.com for you. Been there, done that, shredded the t-shirt.
2) How much of a problem this becomes is a matter of who's inconvenienced, how much, and what alternative contact channels they've got. Generally, though, it's something of a PITA.
Things can work well for a long time. Or not. You're just never quite sure when they'll blow up.
A lifetime of maintenance. Not in terms of hours, but it's something that requires monitoring. Something that will fail at inconvenient times. Something that requires security updates.
And if that's sendmail smtpd, it's not "a few minutes" to configure if you're unfamiliar with it.
It's opensmtpd and my seven-line config took literally minutes to initially write back when I first did it years ago, with no prior experience.
It's required basically no maintenance at all from me. As for monitoring, I check my mail daily anyway and I will notice it if it stops flowing.
A few weeks ago I actually moved and took down my server. I took an old netbook and made it a new, temporary server. Again that took me literally minutes.
Thankfully smtp is a robust protocol so if something fails, you normally have a couple days to fix it before mail starts dropping.
But what do you do when you're using third-party email service and they decide to kick you out all of a sudden? Happened to me with Google. Happened to many other people as well, also with other providers.
I ran my own debian-exim for about a decade until I got tired of the care and feeding. opensmtpd does look suitably low-maintenance and I might pick it if I ever wanted to do this again, but it was released after I gave up being an email admin. I still have the mail domain, but having the same email address for sixteen years means ending up on a lot of spam lists.
You run your own server so you can easily set it up to accept any email matching some simple rules. This means you can give a different email address to every online service you use. If one of them starts getting spam, blackhole it.
I have been running my own email server this way for about 10 years and I have never needed any kind of antispam measure except a simple .procmailrc.
A few minutes to configure smtpd. A few more to update DNS settings.