Both frameworks have various packages that add OIDC support to them so that shouldn't be an issue.
As for beginner - there's quite a bit of learning curve in terms of writing GUI applications themselves. Avalonia is kind of like WPF* but good(tm) and cross-platform. Uno platform is more like Xamarin and MAUI except in a better shape as of today. From technical perspective, Avalonia focuses on consistent rendering on all platforms with Skia while Uno tries to use native controls instead. I generally would not advise using Avalonia for mobile over Uno. In mobile case you might actually want to also give MAUI a try as, despite harsh criticism it has been receiving, it has improved significantly.
* either platform supports various declarative UI libraries (that are either part of their ecosystem or community-maintained), so you are not married to XAML thankfully.
Performance on mobile devices and UI libraries. Avalonia ecosystem is predominantly focused on desktop, which is not the case for Uno. But I liked the fact that setting up and AOT compiling Avalonia templates was a breeze so use it for side projects. For something more serious that has to target mobile, I would investigate how much MAUI has progressed in terms of quality and if problematic, would just go with Uno.
As for beginner - there's quite a bit of learning curve in terms of writing GUI applications themselves. Avalonia is kind of like WPF* but good(tm) and cross-platform. Uno platform is more like Xamarin and MAUI except in a better shape as of today. From technical perspective, Avalonia focuses on consistent rendering on all platforms with Skia while Uno tries to use native controls instead. I generally would not advise using Avalonia for mobile over Uno. In mobile case you might actually want to also give MAUI a try as, despite harsh criticism it has been receiving, it has improved significantly.
* either platform supports various declarative UI libraries (that are either part of their ecosystem or community-maintained), so you are not married to XAML thankfully.