I have worked for financial services companies that used FreeBSD both in EC2 and on the metal in data centers (self managed). The two features we used all the time were zfs and jails. Each service ran in its own jail for isolation. One (not even beefy) server could run all the services which was insanely cost efficient. A cloud migration was undertaken at some point to have a hybrid setup, using a mix of Linux (k8s) and FreeBSD, and costs skyrocketed. It’s a trade off because in the data center we had to buy and replace our own disks, react to fires taking place, being only in one country etc. AWS gives you multi region, and tons of good stuff, and that has a price.

ZFS was not leveraged that much but it saved our beacon once when a table in the production database was accidentally dropped and we could instantly rollback to the previous zfs snapshot (there was a tiny bit of data loss as a result but this did not matter too much for this application - uptime was more important). ZFS was also used for backups I believe.

A few times I used dtrace in production to troubleshoot.

When we introduced Linux to our fleet of FreeBSD servers, every team picked a different distro organically so it was a bit of a zoo. With FreeBSD on the server you only have the one variant.

I still use and like both, but I must say I really like that FreeBSD is a kernel+OS integrated together.