I upgraded QDB to v8.1.0 and I immediately see. This is new (and nice).
Should I follow these is my question or question the recommendation? How reliable is this?
Does it change based on usage and hardware or is that fixed guidance?
The data is fixed, based on what we’ve seen in the wild.
It was always there on the logs on startup, but it was mostly ignored by users, so we decided to make it more evident 
It can cause problems down the line if you have many partitions and QuestDB hits the limits. The guidance you see there is in line with the official docs at Capacity planning | QuestDB
If you hit the limits in the future (it would appear as an error in the logs) you can always enlarge the limit, but it should be fine for most use cases. Upping these limits is pretty low risk and a good trade-off versus hitting instability too soon. Small issues with schema can cause a lot of files to be used (100 columns, hourly partitions, 1 year of data for example) .