I haven't had an issue with coverage. My issues have been with functionality.
The preview feature breaks if you change anything about the cluster.
Updating addons doesn't work if you have to use the overwrite option for ResolveConflicts
Changing the node group size just breaks because Pulumi thinks it can update it, but it needs to replace the node group instead.
Tonight, I'm upgrading some clusters and while it is working, it is not really handling the order of operations properly, even though I have dependencies setup correctly (I think). As a result, I'm having to run update multiple times to get it to go through all the steps.