Every team I've worked on that has switched from Kanban to Scrum has slowed down and never regained the previous velocity. Further, many of the promised benefits of Scrum (things like better insight and predictability about delivery rates) never materialized. Every team that I've worked on that has switched from Scrum to Kanban saw an immediate improvement in speed that never disappeared. Once, just once, I was lucky…
Yes but raw velocity is not the only thing that matters. For some projects it is, but for many the accountability of Scrum + the grouping helps with dependency management across an organization.
Of course, the counter to that is that we're not doing scrum right. The question in my mind then is if I haven't seen a single team do Scrum correctly there's something wrong that makes the methodology so difficult it can't be applied ...