I'm seeing strange UA behavior in Ketarin 1.8.7. I haven't tested this thoroughly, but in my experiments I'm seeing a couple bugs.
First, I'd love for the global default user-agent to be alterable. Not just the UA for specific apps, but for the entire installation all at once.
Now for the bugs:
I assume that the user-agent will be included with all requests, but that's not actually the case. At least during development in the Variables editor, URL requests are sent with an empty UA. Since this is a sure-fire sign of unwelcome traffic, some servers (including my own) block the request with a 403 forbidden. Including the UA with these requests is important.
Now, the default UA is *not* included in those instances. But if you set a custom UA in the Advanced settings tab that UA will be used immediately on Variables URL requests.
The workaround is to manually assign a custom UA for each app that has problems due to UA filtering.
Expected behavior is to use the global UA for all requests, especially those from the Variables feature, except where it's superseded by the app-specific "User agent" option.