From FlatBuffers overview I see this comparison:
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Protocol Buffers is indeed relatively similar to FlatBuffers, with the primary difference being that FlatBuffers does not need a parsing/ unpacking step to a secondary representation before you can access data, often coupled with per-object memory allocation. The code is an order of magnitude bigger, too. Protocol Buffers has neither optional text import/export nor schema language features like unions.
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So are the newer ones useful mostly when serialization vs deserialization speed matters (https://google.github.io/flatbuffers/) ?