1) Uptime (though this could be partially alleviated by retries)
and most of all:
2) "Trust"/"Spam score"
It's the main reason to use Sendgrid, AWS, Google, etc. Their "value" is not the email service, it's that their SMTP servers are trusted.
If tomorrow I can just send from localhost instead of going through Google it's fine for me, but in reality, my emails won't arrive due to these filters.
Even if your "self hosting" is renting a $5/month VPS, some spam lists (e.g. UCEPROTECT) proactively mark any IP ranges owned by consumer ISPs and VPS hosting as potential spam. I figured paying fastmail $30/yr was worth never having to worry about it.
e.g. you spend a lot of money to show that you are a legitimate entity or you pay less money to rent something that shows you are connected to said entity.
But yes, the “trust / spam score” is a legit challenge. If only device manufacturers were held liable for security flaws, but we sadly don’t live in that timeline.
I had quite a bit of success with it and of course, DKIM and the other measures you can take some years back.
For personal emails, I don't think I had any which fed straight into spam.
See jwz's struggles with hosting his own email. (Not linking to his blog here with HN as the referrer...)
With email, the 800 lb gorillas won, and in the end it didn't even solve the spam problem.
Maintenance is probably my number one reason for giving up on projects where I'm responsible for feeding the pet.
(Back then email still worked from residential IP addresses, and wasn't blocked by default)