Cats living totally indoors survive longer than outdoor cats. They are not mauled by dogs, run over by cars or attacked by feral tom cats. The feline AIDS virus (caused by Feline Immunodeficiency Virus or FIV) is transmitted by cat bites.
Cats subject to solar cancers, i.e. white and tortoiseshell cats, are not exposed to ultraviolet rays through the window glass.
Some cats will not be happy indoors and must have freedom with its risks.
If a cat is happy indoors, there are no benefits for it being free outside.
Odorous litter trays may be a problem for the owner but this is often managed by the use of 'premium' brands of dry food.
Scatching posts are almost a necessity for some cats.Make sure it is heavy and very stable otherwise it will frighten the cat and will not be used.