I don't know if this changed in the past month or so, but it used to be that you wouldn't really care if the data for the UI tests changed (other than if the data you've added is displaying correctly or not). See https://github.com/piwik/piwik/blob/master/tests/README.screenshots.md :

Note: When determining whether a screenshot is correct, the data displayed is not important. Report data correctness is verified through System and other PHP tests. The UI tests should only test UI behavior.

True. Although not really convinced the System tests actually verify the data correctness so much ;)

In my case I made a change in Archiving and I had to update the OmniFixture otherwise every screenshot would show an error message. After updating the OmniFixture some screenshots still fail and now I don't know whether it is due to my change or due to any previous change. So what I'd have to do now is basically to go to master and cherry-pick the OmniFixture and check whether the same screens fail which is quite a bit of an effort.

I will simply assume the displayed data is still correct and update the screens. Cheers

Although not really convinced the System tests actually verify the data correctness so much ;)

Maybe not, but I don't think I've ever encountered a situation where you couldn't replicate data related issues in a UI test from a phpunit test (actually I can't remember a situation where the UI test data difference was incorrect). I think the time saved is of more value than catching strange bugs that may never occur in production.