We plan to create a hosted web app with AngularJS. As UI Framework we found Ionic and Onsen to work well with AngularJS. Both of this frameworks promote that they are made particularly for Hybrid Apps. But us I understand correctly, both frameworks are based on web technologies. So what are the drawbacks when using the suggested UI frameworks for non Hybrid Apps? Is it mainly the Browser support?
There is a part of Ionic that expects the Cordova/PhoneGap Device plugin to be installed. It helps with displaying on Android and iOS appropriately, as well as increasing the size of header bars for iOS 7. That wont be an issue for web though, since you'll have the browser's "chrome" taking up that space.
You may loose some of the buttery smoothness in things like page transitions because the template files need to travel over a (often cellular) connection to the internet, but your app will most certainly work if built with Ionic or Onses over the web.
Well, both Ionic and Onses are made particularly for Hybrid Mobile Apps. You just won't be able to use it even for tablet apps without customization (If I am wrong, please correct me) just because they target the limited viewport and incorporate phone UX patterns.
Also, they are made with PhoneGap in mind (the apps will be hosted in the WebView component), and addresses some common WebView problems, e.g GPU acceleration for better performance.
If you need just some UI framework for a simple Web App you'd better have a look at Twitter Boostrap or Zurb Foundation. For a complex case you may even consider Sencha instead of Angular.
Things changed considerably over last few months.
Both frameworks now support splitview so they can be used for tablet devices. Subjectively Onsen UI has a better support (it is much more configurable), but Ionic is also not bad. This can translate to smaller desktop browsers screens.
You should also consider that Onsen UI has a dedicated desktop browsers support (all web-kit browsers). Ionic framework will also work on desktop browsers, but it was not fully tested or optimized for such environment.
Biggest problem here is that both frameworks were never meant to be used on larger screens. They look oversized and bloated. If you ever worked with jQuery Mobile (on desktop browsers) you will understand this problem.
There is nothing stopping you from doing that - it might even be a smart approach for a mobile first design approach. The advantage is that you could rapidly iterate an idea an then invest in a native or platform specific app depending on user feedback/metrics.
The main drawback would be the user expectations for your UI and application logic which are different for a web page and a native app. For example the back button will be duplicated in the browser and in your header. You could use some platform specific CSS or configurations to hide it. Then there is your application state and user data - will you save it in local storage, session or database? This depends on your app and best judgement.
There will be some browser issues which may not be addressed in the core of Ionic and Onsen because they are not targeted to work with IE6/7 (see angular browser support), for example, but you can resolve those by having a graded browser support policy.