From the Hubitat Hub’s perspective, writing data is simply making http calls. This is very simple and allows for any RESTful server to collect the data and do whatever it wants to. You could easily write a RESTful server (that runs on another system, not the Hubitat Hub) that writes CSV files if desired.
Syslog requires UDP (typically) which is not yet released as a user callable API on Hubitat, yet. Syslog is fine for logging text, like trace debug data. But I would prefer not to use it for time-series data.
Most users would rather use something like InfluxDB, as it is supported, fast, free, and works on almost every platform. Paired with Grafana, you can make very nice dashboards and displays.
I work in manufacturing, where we use OSI PI as our data historian. I can tell you from 28 years of experience, tools like OSI PI and InfluxDB are much more efficient at storing and retrieving time series data than simple text files.
Options are good! The nice thing about a platform like Hubitat, is you can write your own app to do whatever you’d like. Just please don’t poll continuously for data. Hubitat allows Apps to subscribe for device events. So, only when data changes is it logged. Data Historians, like OSI PI and InfluxDB take care of filling in the gaps of data, where a value has not changed for a while.