I think developers often want to find a “one true cross platform toolkit for all cases” and I don’t believe that will ever exist. It depends so much on both your app/market but also the skills and experience within your team. No 1 priority is move quickly.
Flutter is brilliant, and probably the way to go, for some apps. A great example of a successful Flutter app is the Sonos app, relatively simple and responsive ui talking to an api. No “rich” or user supplied content. Although they haven’t replaced their desktop app with the flutter app, my guess is that it is the plan.
I think Flutter is brilliant for that type of IOT app, banking/crypto/utilities also the sort that would work well. I don’t think it’s best placed if you want to start using WebViews for some content, when I was last looking at it a year ago there were serious problems.
If you are building a social media app, either Native or React Native (or a combination) is probably the best way to go. You are aiming high so need the best performance and ux.
If you are building a b2b SAAS app I would probably reach for something like Ionic/Capacitor. You can share almost all you codebase with your web/PWA/desktop (electron) apps and can achieve it all with a tiny team. If you need a little more native functionality then adding in NativeScript to a Capacitor works very well.
For an e-commerce app I would probably go Iconic/Capacitor too, the Amazon app for example is mostly webviews.
Productivity apps are much more difficult to recommend for as you probably need large native components.
I've been wondering why webviews with native page transitions aren't a more commonly used thing. It improves the UX a lot by dealing with the abrupt navigation changes of regular web apps. There was a plugin for Cordova that did this (https://github.com/Telerik-Verified-Plugins/NativePageTransi...) but it's abandoned.
You render your mobile web view like normal, wire up a JavaScript handler (formerly known as Turbolinks), and push native screens on iOS. It works really well for CRUD and "boring" SAAS apps with little interaction outside of forms. And when you need higher fidelity dropping down to SwiftUI or UIKit is straightforward.
https://github.com/hotwired/turbo-ios
To make things even simpler, I built Jumpstart iOS, which takes care of all of the Swift boilerplate. Navigation, authentication, and push notifications all work out of the box after adding a few endpoints to your server.