Discomfort
Polyurethane foam absorbs and stores body heat. Trying to sleep on top of a slab of foam during warm summers can make for a hot and uncomfortable night. Memory foam also may eventually lose its ability to spring back to its original shape, causing depressed areas in the mattress.
Odors
A mattress constructed of polyurethane layers can give off chemical odors and fumes. This is especially pronounced with some memory foam mattresses. Some people may simply find the odor objectionable, while others may become ill with prolonged exposure.
Durability
Cheaper polyurethane mattresses can gradually disintegrate and lose their quality of support as they age. Inexpensive models without memory foam may be light and insubstantial with no edge support, skittering around as pressure is put on one spot.