Not sure what your friend is referring to, but packages with exposed dies are definitely more fragile than ones with built in heat sinks.

One chiplet probably can't be sanely replaced if it dies, but equally we couldn't really cut out and replace part of a single die either. So that seems like a wash.

I could believe they're more vulnerable to mechanical damage. Also seems possible that the thermal expansion introducing mechanical stresses is more of a problem. I suppose we won't really know for a while yet.

This is what I would expect. With the thermal conduction across the planar barrier being different, disparities in heat flow to the sinks and the possibility of one chiplet running hotter than the other parts, there might be too much stress on the joined area. This is also assuming that the package is not experiencing any additional mechanical stress from motion or vibration.