Let me restate what you're saying to see if I understand it:
A product that looks the same across platforms is better than a product that adapts itself to the platform it is on, in part because platforms have a lot of UX that isn't good.
My argument for why its better to adapt a product to the platform is threefold:
1. Users have made the choice to use a particular platform and either like or at least tolerate that platform's approach to UX.
When in Rome.
2. Users are using many apps not just the one you made.
This is about scope. You can't just optimize your app in isolation, you have to take into consideration the entire scope of usage on a platform. It's certainly possible to create the perfect UX for a given set of tasks a user has to perform in an app, but drop that into a platform and its likely you've just added a huge learning curve and ongoing cognitive load.
3. Users mostly do not use your app on different platforms.
Users probably own either iOS or Android. If they own one, they aren't even aware of how the other one works. And on desktop, who cares because screen size and pointing device differences necessitate different UX anyway.
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I think there are some instances where having the exact same UX on very different platforms is more ideal UX for the user, but only after it's acknowledged that a) its not just a proxy argument for being easier on the developer/designer, b) its not just the result of developer/company tunnel vision which leads to the assumption that users care about the app as much as the developer/company does, c) users actively want a different experience (e.g. immersive game), and/or d) users are moving more frequently between platforms than different apps on platforms (e.g. a business use case).
From a business perspective it's certainly fine to choose to use a cross-platform kit as there's many reasons to do so, I'm just arguing that it's not ideal for the user the majority of the time.
As far as whether platforms have not-good UX, I mostly disagree. I'd need to see examples of what you're talking about. If by swipe scroll you mean momentum scrolling I disagree. That has great utility on small screens to traverse large lists and it grounds the device in physical metaphors. It's a fantastic UX innovation.