I've been down this road: it did not end well. Facebook also went down this road and seems to have realized something similar (and hence why they pulled back on HTML5 on mobile and have not attempted to make React Native fully cross platform).
The reason why is essentially every platform (iOS/Android/web) has its own characteristics, charms, quirks, habits, and behaviors. Trying to smush these all into one unified UX experience is producing something that feels and acts weird on every platform, and people notice. Trying to unify is a solution for a developer problem, not a solution for a user problem -- and thus it's not really a problem.
What this comes down to is focus. Developing across multiple platforms is a hard problem and requires a lot of work. It's really only suitable for larger teams. As a small team, it is better to focus on one platform, do really well, and then later when you can afford it, go to other platforms.
I kind of disagree, the argument back then (6 years ago) was that the rendering was slow. That argument has not been revisited since. And new ones were made along the way. IMHO, Facebook was just looking for developers mindshare. If the core of your app is bloated, speeding up rendering is pretty moot.
React native to this day is not performant. On iOS the difference may not be as noticeable for most UIs as most iPhones have pretty powerful processors, even for devices 3-4 years old. However, on Android, it’s a crapshoot.
The reason why is essentially every platform (iOS/Android/web) has its own characteristics, charms, quirks, habits, and behaviors. Trying to smush these all into one unified UX experience is producing something that feels and acts weird on every platform, and people notice. Trying to unify is a solution for a developer problem, not a solution for a user problem -- and thus it's not really a problem.
What this comes down to is focus. Developing across multiple platforms is a hard problem and requires a lot of work. It's really only suitable for larger teams. As a small team, it is better to focus on one platform, do really well, and then later when you can afford it, go to other platforms.