Besides tech debt, a concern I have that I don't see brought up is burn out. With Scrum, every action you perform is micromanaged and with a push for "high velocity". There is no proverbial breathing room in this where the pressure lets up. At least with waterfall (for how we did it before Scrum), the windows of high pressure times were shorter. During the beginning of our 6 month waterfall, in parallel to spec work we'd be taking care of tech debt or implementing our pet feature and it was a time of mental recovery.