The number one complaint about dog beds for large dogs, by far, is that they flatten out over time, especially in the middle.
Even worse, some beds flatten out immediately, causing tender elbows and joints to hit the hard floor the first time your dog flops down. To make matters worse, a flat bed is harder for your dog to lay down on and to get up from, particularly older dogs with arthritic joints.
Since giant dogs will often sleep 18 – 20 hours a day, this is a very valid complaint. The constant pressure of a 100 pound plus dog will invariably flatten out a poor quality bed or one made of non-supportive materials. The result is a thin bed that offers no real support and can cause or aggravate the orthopedic issues that all large and giant dogs eventually have.
The worst material for flattening is probably the cotton type fillers. These are very common in lower cost and poor quality beds. Amazon is filled with consumer complaints about these types of beds, even for medium size dogs, much less the larger breeds.
High end poly fill is better, but will still eventually shift around and flatten out over time. Complaints about flattening with poly fill beds are less but still very common.
The best protection against flattening is to buy a bed made of true memory foam and the thicker and heavier weight, the better. Look for four or five pound weight for the most resistance to flattening.
Even with true memory foam, flattening will still be a problem if the pad is too thin. For larger and giant breeds, at least five inches of quality foam is needed. This will not only prevent flattening but the extra height makes it easier to for your large friend to get up and down.