There is a vast difference between the complexity all up you are and can do with Agent Flows and Tools, then you can do in a Topic respectively in the context you are talking about.
Its essentially Apples to Oranges.
If you are simply using things like
Send an Email, that is a single action
If you are wanting to send an email, us AI builder, create a prompt bla bla bla, then you are looking at building a more complex flow than trying to fit all the puzzle pieces together in a Topic.
Does it cost? Sure?
But in most cases or at least a lot of them, when using Topics, you are referring to Conversational things, (mostly), not autonomous agents, triggers etc.
So in the end, the answer is, it depends on the exact scenario you are building. Generically asking if one is better than the over, with over simplified scenarios just reducing it down to a non-answer answer.
If these suggestions help resolve your issue, Please consider Marking the answer as such and also maybe a like.
Thank you!
Sincerely, Michael Gernaey