Most of the account types do status-syncing more often than feed-refreshing.
With Inoreader it’s slightly different — it does some status-syncing outside of feed-refreshing, but less than the other account types, due to Inoreader’s API quota.
On the Mac the app can be running but not the frontmost app — that’s fine. It still syncs.
On iOS, if the app is not the current app, then it may wake up in the background and sync occasionally — but that’s up to the system scheduler.
I use iCloud for syncing since it’s the system built-in to NetNewsWire.
iCloud is not like the other systems in that iCloud is a general system, which means we had to build our own support for RSS syncing on top of iCloud. Which means there’s a lot of code we’re responsible for, so I should run it myself.
As far as the other systems go, I can’t play favorites — I don’t think it would be fair to them. Based on what other NetNewsWire users say, though, they all appear to be quite good.