Yeah, I think the "simple things easy and hard things possible" is a good way of framing this. I've written a blog post about the design philosophy of Mesop and how it differentiates with existing frameworks:
https://google.github.io/mesop/blog/2024/05/13/why-mesop/
Re: flet - flet intentionally takes a more imperative approach whereas Mesop embraces a declarative UI approach (as popularized by React, et al). I think the declarative UI approach scales better as apps get more complex, but I think the best way to see the difference is to look at some of the examples and see which approach you like more.
Re: nicegui - at the high level, there's definitely some similarities with the component-centric UI model. The data binding/state management works quite a bit differently and Mesop's approach provides stronger type-safety and, IMO, is more Pythonic, but that said, there's a lot of good options in this space and I think it's neat to look at all the innovation that's happening here, and ultimately, some of this is based on aesthetics and how you like to code your UIs.