Nowadays Debian 10 is stable release and Debian 9 became oldstable. Certainly, Debian 10.1 Buster provides everything out of the box to meet the Nextcloud server installation requirements. 
There may be good reasons to use FreeBSD (including but not limited to being a U.S. citizen). However, there are good reasons to use Linux (including but not limited to being a EU citizen). 
Unfortunately, one cannot ignore tribal aspects and any community effort can degrade into lost debates or other complex issues. Naturally, technical and deployment aspects should always matter most and you are free to choose.
The Debian project is a strong supporter of free software. 
Have a look at https://www.debian.org/
Please be aware of the Unattended Upgrades to keep the computer current with the latest security (and other) updates automatically. 
IMHO there was never heard of a “death march to upgrade” from the more conservative subset of the several Linux distributions or prove me wrong. Please note for a lasting time the consecutive updates and several migration to quite a bunch of machines (iron and/or VM) from Debian 8 and Debian 9 to the recent Debian 10 gave one no true hassle ever, if I may.
Hope this helps. 
Happy hacking. 