Nostr breaks down along the longstanding ambiguity around what 'decentralization' means.

In the literal sense, centralization leads to economies of scale and the alleviation of coordination issues. Those coordination issues are what make physically decentralized networks so complicated, inefficient and fractured. These networks usually just find ways to centralize despite.

People really want decentralization of power—let's just replace the word with a better word: accountability. One should be able to enjoy economies of scale (centralized infrastructure) but have the cryptographic mechanisms in place to ensure the infrastructure must reveal its use and abuse of power, and can be easily replaced.

Nostr, like most 'decentralized' tech will switch between the meanings of both. Either way, it fits the description above: a single relay can work well, multiple have awkward coordination. The sense it which it is accountable is only that it can be replaced, but network effects favor the largest, so that's a problem.

What "awkward" coordination? Clients fetch updates from whatever servers they follow.

Same thing with email that you've been using for decades, doesn't get simpler than that.