I've kept an eye on Kivy for quite a while and prototyped a bunch of projects over the years with it, and I'm not a big fan.

It feels like it's at the wrong level of abstraction for... basically everything.

The Pong game demo is an example of this: if you're writing a Pong clone (or most video games) where you're going to be operating on a canvas, you don't need all the widget infrastructure that Kivy offers--you're better-served by something like PyGame.

On the other hand, if you want to build a UI using standard widgets, the widgets they provide out of the box aren't particularly fully-featured or even good--you end up doing a lot of hand-coding of functionality that could be included, and the defaults aren't particularly desirable, so you end up having to configure a lot of, for example, visual display settings.

As another user pointed out, their default widgets don't support accessibility meaningfully, and there are many other features, such as dark mode/color scheme support, which modern users expect and which you'll have to code yourself. Realistically, a lot of clients aren't going to give you funding for accessibility features, so the defaults are what most projects will end up with, and if it's me developing it, I'm doing accessibility on my own time, so I'd want this to be configuring what's largely already there, as opposed to what Kivy has: implementing it from scratch. In 2024 I'd view failing to support accessibility reasonably out of the box, as almost a moral failing, and certainly this is enough to discount Kivy from being used for any product intended to go to production.

There IS a fairly vibrant ecosystem of 3rd-party widgets (flowers; there's a "garden" metaphor in their branding for the ecosystem). But this comes with all the problems of a 3rd-party ecosystem: Kivy itself is probably large enough that it won't become abandonware in the forseeable future, but 3rd-party projects aren't, and there are large security and reliability risks to pulling in a bunch of small packages maintained by developers of various talent, intention, and funding. These are risks you generally have to accept for something unusual, but you shouldn't have to accept these risks for your bread-and-butter widgets like buttons and dropdowns.

If you're embarking on a project that benefits from using pre-built widgets like this, the framework I'd recommend is Flet. My experience with it has been overwhelmingly positive, and I've entirely switched away from PyQt for any new projects. The one criticism I'd give is that it doesn't really support multi-window, but that's something I'd avoid for most projects because multi-window support can never really be cross-platform, since mobile platforms don't really support windows as such.