Think of orbitdb as a git repository where everyone has write access. Malicious nodes will spam hundreds of gigabytes of data into it until it's large enough nobody can clone it. Even if you solve the spam problem you still have the problem that you need to download the constantly growing dataset. The blockchain is already 160GB big even though it's transaction throughput is anemic.

Full p2p applications can't offload computation, computations have to happen on your computer and for computations you need the entire dataset. This is fine for messaging and static files.

Federation is a far better idea. You get the benefits of centralisation and decentralisation.