My biggest problem is that, something like a third party browser can't be more than a re-skin of Safari. Apple's App Store guildelines require something like Firefox for iOS to use WebKit, and doesn't allow add-ons period. In this instance, I would argue the default browser can only be
skinned and not changed outright.
It's the same gripe I have with iOS keyboards, unless something has changed there. The feel and layout of every third party keyboard is the same as the default keyboard. During my short period with an iPhone I gave Gboard for iOS a try, tweaking it as much as I could to get a similar experience to what I was used to on Android. In the end it still felt like the system keyboard more than anything.