I'm not sure if a cross-platform UI framework is a good idea or not. I've been building with QT for 15 or 20 years now, and I'm still not sure if cross-platform framework is a better idea than just writing a different UI as a thin layer on top of your business logic for each platform.
Certainly, if you write a native UI for every platform, you're going to need a few more people because you have to have someone who understands each platform. However, with a little bit of work you can define some good interfaces to that UI and most of your needs are going to be things that are not the UI framework but they're logic that can be cross-platform.
What cross-platform frameworks give you more than the UI is all the other pieces you need. I've written a bunch of applications in Qt Core with no UI at all because it's so convenient to have those Qt abstractions.
Certainly, if you write a native UI for every platform, you're going to need a few more people because you have to have someone who understands each platform. However, with a little bit of work you can define some good interfaces to that UI and most of your needs are going to be things that are not the UI framework but they're logic that can be cross-platform.
What cross-platform frameworks give you more than the UI is all the other pieces you need. I've written a bunch of applications in Qt Core with no UI at all because it's so convenient to have those Qt abstractions.