For the reservations, it doesn't really make a difference. Some websites want a pass cover number to book a reservation, which is only on the paper pass cover. But if you've bought a mobile pass, then you can request a pass cover number here.

You'll find a comparison between the paper and mobile passes here.

I see you're from Germany. If you decide for a paper pass, you can also just buy one from any Deutsche Bahn ticket office. Then you have it immediately and you don't pay extra for sending it via the post.

The paper pass: you need to state start/end dates right when buying, on mobile you can decide when to start by simply using first day. This month there is a 10% discount, also when buying at a DB-Reisezenter.

I follow this forum since a few monthes and have seen an enormous listing of all types of errors and problems, more it seems with mobile passes. Could simply be due to newness. On the other side, it seems that SOME of these precious REServations are easier via mobile and direct link, but as I am not the tipical newbee who only seems to ride a few hard-to-get TGV trains, I mostly use other trains in other countries.

Nope, the app just forwards you to the Interrail website that you can also use when you have a paper pass.

I still prefer paper you have something in your Hand as  proof and not all conductors are trained for the new mobilepass (there are even rare cases that a conductor never seen a Interrail or Eurailpass before)