What is your issue?

In dataset_update_method we call merge_core without specific values for join and compat.

These values default to join="outer" and compat="broadcast_equals"

We should probably migrate to join="left" and compat="override". Using override will avoid any potentially expensive equality comparisons for non-indexed coordiante variables. I am not sure how compat="override" interacts with priority_arg.

cc @shoyer