Is ZFS stable enough on FreeBSD? Seems to be an unofficial port.

ZFS is very stable on recent versions of FreeBSD, I can't speak for older versions as I don't have experience. I at least haven't had any issues with it and we use it in production.

It is an older version and not as feature complete as what you will find in Solaris.

ZFS is very stable on FreeBSD.

But you need a lot of extra RAM compared to the usual filesystems. Is that correct? If yes, is using ZFS worth the cost of the extra RAM?

You don't need the extra ram. ZFS uses it as a cache, more ram, more cache. That's about it, otherwise it doesn't use much ram.

ZFS doesn't use that much RAM if you don't turn on deduplication, which is of less use in a system that serves large compressed video files than one that handles tons of read/write traffic as an office file server.

RAM is not required; it just makes lookup faster with caching.

Why would they use ZFS on a CDN? IIRC, ZFS has some performance characteristics that would make it a poor choice for something like caching data on a network edge when it's really designed for ensuring data integrity.