The kernel decided to have a different uAPI on different architectures. This is not just struct size that might differ, but the fields available and their possible values. Imagine a JSON field being a numeric constant in one arch, but a string in another. I get why they did it, but it certainly causes... surprises.

I imagine the issue you experienced might have been either that you hardcoded the numeric size instead of using sizeof and then building on a different architecture, or that you redeclared the struct for one architecture, ignoring that you were building for another architecture.

Not to undermine the annoyance or surprise-factor of your bug, I am tempted to say that it falls into generic logic bugs rather than the fault of any system. Changing to an encoded with dynamic length for example would just introduce different classes of logic bugs, like the whole series of possible issues within JSON parsing/serialization and requiring dynamic buffers.

A struct on the other hand only requires referencing the right struct declaration - the only amount of care you need is to include and use them. In my opinion, this is the maximum possible convenience for such system.