The main difference between modern plush and ultra-plush mattresses lies in their firmness levels. Plush mattresses typically have soft foam and rank around three on the mattress firmness scale, while ultra-plush mattresses are at the end, ranking 1.
Ultra-plush mattresses are the softest beds available, but these soft beds may lack proper joint support for most sleepers. Additionally, the extremely soft nature of ultra-plush mattresses can create a "stuck" feeling for some individuals. It's worth noting that many companies do not commonly offer ultra-plush mattresses, as they are unsuitable for most sleepers.