always $set usage with updating from commit

[Suryansh2002]

it seems umongo always uses $set to update , which means if i want to remove a key from a dictionary field . it sets the field again , instead of unsetting the key from field ,
so i was wondering if there are any performance drop compared to using {"$unset":{"field_name.key":1}}
I didnt knew where to ask ,so i m here!