Although an expanding puller works in most cases, if the post is in there a good way and its properly oxidised together, the slide hammer puller just distorts the alloy.
I've tried.
How about the classic where a PO has hammered a stuck post down into the frame?
That's a lot of fun
Usually it's not gone far enough to allow decent insertion of the new post, sometimes its split the seat tube where it wasn't reamed.