The thing about these relatively small drum sanders is the infeed and outfeed rollers that hold the work down don't really exert much pressure like a planer does, and therefore they don't force the stock flat to the table.

Can a drum sander replace a planer?

Drum sanders are great for fine surfacing or sanding wood, especially figured woods that tear out under planer blades, but it is not the best tool for removing large amounts of wood, such as you would do when leveling glued up boards or surfacing stock." ... A drum sander is not made for the rigors of abrasive planing.

What is a drum sander good for?

A drum floor sander, also known as a belt sander, is a heavy-duty machine designed for heavy sanding of solid wood flooring. A drum sander cuts through the wood, obliterating multiple layers of wood as well as any varnish, giving you a new surface to refinish.