This has happened to our cat who managed to scramble up to a shelf in front of our big bay window in our living room. He was trying to swat at bugs that will occasionally bump into it from outside. Our shelf is twice as wide as this one, and he still lost his footing :/

Ours isn't as high up as this one, thankfully, and we keep a cushioned rug and dog beds directly underneath it that he landed on, so he wasnt hurt. But yeah, this one would make me nervous.

'Course, they know their cat's disposition best, maybe they're super chill and never move very fast/suddenly. Would just hate for someone to try this and have their kitty get hurt