Dentures are made out of plastic acrylic. They can have clasps added which hook around your remaining teeth to help keep the dentures in place. Dentures can also be made out of metal. These dentures are much stronger and easier to tolerate as they cover a lot less of the roof of your mouth.
They are more secure due to their clasps and rest seats and conduct heat thereby allowing more appreciation of your food. They are more hygienic as they cover less of your gums allowing your own natural saliva to cleanse these areas.
Even a well fitting lower denture can be loose. If the teeth have been lost a long time ago the lower ridge which the denture sits on can be very 'flat' so the dentures have nothing to grip onto. A lower denture relies on the user controlling it with their lips and tongue. For many people this can be a struggle initially but can be overcome in time.