glad to hear you find the tools useful! Let me try to answer your questions.
One problem which you might be facing is that particular PrimeFaces element is not physically visible in the browser (speaking from the user perspective). If it does not fit the viewport you can face this exception. More details can be found here in the WebDriver spec.
Is there any tutorial that exceeds the basic information from official that covers how to write a code for simulating user operating Primefaces components?
Not that I'm aware of. mjobanek are you aware of something? On the other hand we would be extremely happy to see contribution for this If you could help us in this area we definitely can provide you all the details on how to write such a guide! And all the glory is yours
Is there any problem with injecting page fragments inside page fragments ? (for example I have a <p:tabView> that contains multiple tabs and then <h:form> on every tab - can I make page fragment class for tabs and then FindBy the forms inside ?
Are you experiencing any problems with it? For me that is the main killer feature of Graphene which let me write UI tests in a very concise and clean fashion in all the projects till date.
sorry for my very late response. No, I'm not aware of any such a doc or guide.
If you have any thoughts or notes on this which you could share with us, we (and many others) would be very thankful for them.