Considering that the Raspberry Pi 4 recently gained support in Nextcloud Pi, I say that the Raspberry Pi will be a pretty tough act to follow for a cheap, decently reliable SBC for running Nextcloud on. Especially when you attach an SSD in an enclosure to the USB 3, and run the SQL DB there, as well as store the data dir there (and the ncp utilities make it easy to move these).
“NextCloudPi gets RPi4 support, a backup UI, moves to NC16.0.3, Buster, PHP7.3 and more”:
https://ownyourbits.com/2019/08/05/nextcloudpi-gets-a-backup-ui-moves-to-nc16-0-3-buster-php7-3-and-more/
PS: I recommend buying a 2.5" SATA-to-USB 3 enclosure which has either a JMicron JMS578 or JMS567 controller chip, if you can find it. Those are known to work really well in Linux.