All these articles really need to clearly layout the downside. You can't send emails as the custom domain. At best, your emails will say "sent by x@gmail.com on behalf of officialdomain.org"
The "via forwardmail" part is subtle enough that it's not a big deal for me and if you start looking out for it, you'll see it a lot.
EDIT: I was trying out Cloudflare email routing yesterday so I checked one of the test emails I sent. There's no "via Cloudflare" visible, and actually it's sent from smtp.gmail.com so that's probably reputable enough to get high deliverability. All I see in the from field is "abc@mysite.com".