I have trouble with some of this jargon so I apologize if I misuse some of it.
In my past experiences Story Estimation (planning poker estimating how difficult/complex certain tasks will be with fibonacci numbers) happens before Task Breakdown (figuring out the implementation details of how the team thinks we should implement the storys that have been estimated and slotted for an incoming sprint).
I feel that doing Task Breakdown before Story Estimation,would be advantageous for junior or new team members especially if the codebase is large or complicated.
But I feel story estimation would be easier and more accurate if Task Breakdown occurred prior such that the team has discussed how the would like to implement something, and thus understand how "hard" it is. But I have never seen this.
What are the disadvantages of doing Task Breakdown before Story Estimation?
Story Estimation could be "tainted" as the the task has been broken down?
Planning could become trickier as you will need to breakdown many more stories to have them ready for estimation to be considered for an incoming sprint
Participants can influence each other's opinions on how difficult a task is (for story estimation if it occurs after task breakdown) during task breakdown