Why the netcode issue exists, and why it cannot be fixed.

[Diraco]

It was equally terrible, but the gameplay was so slow, you could play with like 500ms and never notice.

Playing white mage on 11, even with 80% fastcast, I don't remember frequently seeing a cure land on a dead player -- the server cancels the spell before it completes if the target dies while casting (even if the spell takes less than ~100ms to cast). It seemed that any amount cured showed on the client did accurately reflect the server's state. Really the only questionable syncrhonization issue I've seen is something that SE ninja-patched in a few years ago involving zoning: previously a warp or zone change would be successful even if you died slightly before the warp resolves, and you'd end up dead at your destination. Now if you die even while warp takes effect, it's canceled.