I was a big Rammstein fan back in my teens. Though I never got to see them play live, I did seek out a ton of live performance videos, because their live stage show is so wild. That said, I would be very surprised if their whole show was pre-recorded. If you listen closely, there are plenty of little imperfections (or real humanness) in their live performances and clear difference in tones of the instruments on stage VS. those on the album recordings... which I suppose could be intentional to make it seem NOT pre-recorded, but that seems to defeat the point of faking a live performance. No part of their music is difficult to play. Even as a beginner I was able to learn their guitar and bass lines. The real attraction to their music is the heaviness of the riffs and the very growly low vocals, not any kind of complexity.
Regarding pre-recorded music in general though -- I think any musical aspect should actually be performed on stage or just left out of the live performance. At least, if I am paying for a ticket, I will most definitely be disappointed if there is a backing-track of any kind. I understand that there may be elements of a recording that can't be reproduced on stage (like symphony orchestra or choir accompaniment) but come on! You're an artist right? Be creative and work within your means! I would much rather that part be adapted to one of the instruments actually on stage or just leave it out completely. As soon as you introduce a recording of an instrument into a live performance, it creates an artificial structure for the song. I think it just cheapens the live performance, like musical training-wheels. The exception here would be sampled sounds that are being implemented artistically/tastefully, like the cash register in "Money" and things of that nature.
While living in Nashville back in 2013, my housemate was a professional hired-gun drummer, who incidentally had played with some pretty big names in the country genre. He invited me to play bass on a pretty well-paying, recurring gig. Easy bass lines, local area gigs, fun group of guys, etc. Unfortunately, I had to turn it down when he said that the band performs to an in-ear click track. Sorry. I won't do it. I can't help it, but that is an artificial element of live music that I want no part in... I don't hold it against anyone who does it (unless I paid for a ticket). I realize a lot of people probably don't care, but it just feels wrong to me personally. A musician who is being paid to play, should either be capable of playing in time and in tune on their own accord or be honest and just let the performance be what it is.