If I understand correctly any WebView's contents is completely readable by the app it's embedded in. Which means you should arguably never trust any app that opens it's own WebView for view links. For example if you use the QR-Code Reader App it opens QR codes that leads to websites into it's own WebView rather than just forwarding the link to Safari. So, if the QR code is for anything that asks you to login the QR-Code Reader App could be reading your password.
In other words, WebViews are not adding security they're removing it
Otherwise, Apple can claim perf and security but that's arguably false. Chrome on desktop is faster and more secure than Safari by nearly every measure. There's no reason to believe the same wouldn't be true of a real Chrome (not embedded WebView) on iOS
A big issue comes from potential 3rd party browsers not being hardened to browser exploits which could circumvent device security. It may not be able to break out of the browser's security sandbox but if the browser has access to your camera, photos, contacts, etc...
They cite performance and security control, and really, can you blame them?
The web is quite the wild west nowadays, and the WebViews they provide, while not ideal, cover most of the cases while offering a decent level of protection.
And I think that somewhere down the road Apple will open up, when the system is ready.