Latency is likely higher/less controlled with a p2p solution and mobile devices can't easily be p2p nodes unless they're on wifi.
Despite this logic, they were capable of very impressive P2P offloads during the world cup. Their datasheet linked at the bottom of this blog post is worth a read for more details: https://blog.peer5.com/what-a-world-cup/ (email address needed)
I'm curious on how well that works in practice.
I'd imagine there's more failures modes versus a regular CDN even after the initial decision of P2P or CDN. For example, you're downloading from another user who suddenly closes their app or whose network connection degrades. You can handle that but you'll need a larger buffer, on average, which increases latency.