Risk of being blocked by big players (gmail/O365) is real but IMHO very low if you follow BCP. The main problem is that big companies make it very hard or impossible to contact them to resolve the issue.
Can confirm, only issue I have is with outlook.com because their MTA-STS validator is… for lack of a better word: wrong.
I decided that replying to mails from outlook.com is not part of my mailserver's offerings and if it's required others channels have to be sought out. I'm not going to violate established standards just so Microsoft can do things their way.
I'm sure most people here know about DMARC but there are still a lot of mail senders which either have no DMARC record, or send some messages in a way that either DKIM or SPF or even both fail, or have a syntax error in SPD/DKIM/DMARC DNS records.
On top of this it is good to ensure that your servers doesn't generate email backsatter when possible. Send rate limits is a simple but rarely enforced by small servers way to limit amount of spam which can be send if user credentials or web-app hosted on the same server are compromised.