That is not necessarily true. Perhaps it is with the way you use your drum sander. I use mine often when neither side is flat. I don't usually sand long boards or wide panels but mostly use it for smaller things, fairly short rectangular boards, or cut round or to some other shape. One example is these turning blanks, used to make the things like shown in the second photo.
IMG_7484.jpg penta_platter_Kristina_finished_comp3.jpg
I cut these out of 6/4 and thicker boards - they are usually 6-16" across. Most are rough sawn, sometimes cut with a chain saw. Some start out with significant cupping or other warping. My goal is to have two flat and parallel sides, easy with the drum sander. Sometimes I do this so I can better see the surface and the figure any any defects before I decide exactly where to cut a blank. I usually do a dozen or so of these at once, feeding continuously, varying the feed angle often.
I have the Performax 22-44 and use coarse paper when flattening for lathe use.
The wood can start out seriously cupped or twisted. Yes, I know I can mount and turn a warped piece of wood. But I choose to flatten these because that's the way I like to work. Besides letting me better examine the wood I want at least one side flat since I initially mount with a Glaser screw chuck and I want it tight against a flat surface.
If cupped, I put the concave surface down against the belt and run the wood through enough times to make a flat surface on perhaps 1/2 of the convex side, maybe more, maybe less. Like others, I make very small increments in height reduction. Then I flip the wood over and run it through until it flattens part of both edges on the convex side. After that, it's a series of passes with small thickness changes, flipping the blank as needed until it is flat on both sides.
In the case where the wood is significantly twisted it's easy to slip (or glue) a thin shim under one corner to prevent rocking while making the other side flat.
In addition, I often give these blanks to friends and students and when I auction them at demos (proceeds go to the club), a nice, clean, flat blank is appreciated and perhaps even more valuable.
With short pieces it's easy to get snipe if I allow it. I never get snipe on the leading edge and get no snipe on the trailing edge unless I let the front of a small, thick block rise up a bit as it comes through the sander. To prevent this I hold the leading edge down with a finger as it exits the sander. I've flattened warped pieces as wide as 16" and as short as 6". I would NEVER try flattening a short piece on my planer. BTW, you really need one side of a board flat when using a planer too. The usual recommendation is to joint one side first, difficult for a wide board for most people. Hand planing is another option.
I have never had the end of the sandpaper loosen during use. Makes me wonder if the end is properly secured in the tensioning clamp. Perhaps too much force on the drum contributes. Perhaps the spring on the clip has weakened.
I usually run the speed fairly high with coarse paper and rely in the machine to slow itself down if necessary. I don't know if they all drum sanders do this but my 22-44 senses the motor currents and will automatically slow down if decides the machine is working too hard.
JKJ