I was on a delayed ICE, and they handed forms to every passenger. I found the process straightforward.

I don't live in Germany though, so I've only had the single delay.

I've gone through the process some 4 to 5 times in the past two years or so and never had any issues with it. The delays are annoying, but I get part of my money back no problem.

It's great. You can do it in the app now and get money back fast. My colleagues also book exclusively late night trains (which are often delayed) so they get a free overnight stay in a hotel.