I think the reason people don't like Kanban is there are a bunch of tasks that suck to get stuff over the line and without the "deadline" there's little incentive to help with them instead of just pulling something else off the backlog.
Take this with a grain of salt, because I haven't done Kanban personally, but I think the theory is that if you hard cap work in progress, and everyone is sitting around on their hands, it puts a lot of pressure to eliminate bottlenecks to progress.
