I've been using those straps for many years and haven't experience any issues with them. The locking mechanism works fine and no guitar was damaged by falling down with them.
The only downside I can highlight is the quality. It feels cheap. When I bought those straps many years ago some of the stitching on leather ends was torn, so I guess the quality isn't consistent. However newly acquired strap from Thomann was perfect, so I hope the issue is resolved.