There's great ones (like Foliate or Moon+) which are limited to one platform, there's ones that are cross-platform, but look so dated that I couldn't seriously use them for reading (like the one integrated in Calibre), and there's ones which are both fairly modern-looking and cross-platform, but not open source (like FBReader).
If this one syncs my books, reading positions, and highlights across my devices, that's gonna be a huge win in my book.
Might also argue that nov.el³ is high quality and cross platform, but I'm not sure my definition of cross platform would necessarily align with other peoples ;)
¹ https://packages.debian.org/trixie/fbreader
² https://metadata.ftp-master.debian.org/changelogs//main/f/fb...
Like everything else in Calibre, it has all the features I want to have and then some, but actually using them is always just a little bit more complicated than it needs to be.
I know it's annoying but it's not like it's impossible, right?
What exactly is so hard about porting the existing GTK app? That work already exists, anything in Tauri is reinventing the wheel.