From my discussions with Raspberry Pi, and with a few companies who integrated CM4/5 into their products, the board to board connectors were a massive pain to automate in a production line (not necessarily soldering the connectors, but in inserting Compute Modules.

It's surprising how good human fingers can be at getting the alignment, the push, and the slight 'pop' and the feedback required to know when it's seated properly.

That, mixed with requiring extra standoffs and screws to secure the CM to boards for any kind of vibration/mobile use cases probably informed the decision to go to castellated / solder-on.

It's not as friendly for quick swaps or upgrades, but it also reduces the total board height when it's all put together.

I wonder how robust the solder joints are for castellated boards. I’d still imagine that to be a weak point vibration-wise. Definitely easier to automate, but would it be that much more robust?

Thinking about those CM sockets and I think the answer is yes - a castellated solder joint (is that the right term?) would be stronger. But other sockets might be more robust than the CM0.