I use the iCloud custom domain for my personal email and I can confidently say it's worth the cost.

I am definitely engrossed in the Apple Ecosystem™ though so I can't speak to how it would fit any other setup that isn't almost entirely Apple-based.

iCloud plus has had some issues in the past, they pretty aggressively filter emails to the point of just not sending/receiving messages with certain phrases. https://forums.techsoup.org/t/icloud-email-system-blocks-ema...

Unlimited addresses is not something iCloud+ offers directly.