Safari is already very similar to chromium. In fact a lot of chromium is already built off of early web kit.It's bad because it allows Chrome/Chromium on iOS devices which means there is even less push against web developers to support Safari/Webkit.
This will hurt me as a Safari users on Mac and iOS.
The issue with a single browser engine, is that if there’s a vulnerability. You’re stuck until that 1 company decides to fix it. You don’t have any alternatives in the mean time.