On the Table, the dropdown defaults is always changing after the last selected action.
I would default to the most frequent action. In the example shown, I wonder if changing a password is the most frequent action taken. Just because I changed my password yesterday doesn't mean I want to do it again today.
Can you give some more information about the context of this dropdowns? What is the impact to select an option and whats the action behind this.
Normally there is no disadvantage, provide you put a descriptive label above or on it. If you dont do this it is difficult for the user to see why he filled the field and what the intention behind it was. Especially in long forms, which the user has to fill many of these fields, it is complicated and triggers a high cognitive load for the user.
Much in agreement with Eric of defaulting to the most frequent action.
There's a lot to be said about muscle memory. If the top action changes all the time, the user is forced to read the label every time to confirm it'll do what they want it to do.
Having a static placement of options may involve an extra click, but you can go pretty fast once you've learnt the location.
That said, if there are 10+ rows of data to change at once, you probably want to provide some sort of bulk action option for the user.