What are disadvantages of wood floor covering in shower?
There are woods used in wet environments, mostly those that are fairly impervious to rot - teak, cedar, redwood, mahogony (less so).

Untreated: Sometimes cedar, redwood and teak are left natural. In almost all these cases, they are not exposed to prolonged soaking.  Even in saunas they are misted and then dried, not soaked the way a shower soaks. On Boats they get spashed, they dry.  When they are left untreated, the wood weathers, loses its color and eventually becomes somewhat rough. Cedar and redwood are fairly soft, and on a floor will wear out, probably quickly. Teak will last longer but will roughen up and wear out eventually. Also untreated wood tends to hold water and is more prone to developing mold or organic growths, such as bacteria, especially in a shower that is not exposed to sunlight.

Treated: These woods can be finished with a penetrating sealer or a hard marine finish, like spar varnish or the harder polyurethanes, or even an epoxy finish. Even then, these finishes need to be redone often, sometimes every year, depending on the amount of soaking and other wear.

Finished or sealed Wood can be used in bathrooms where there is just an occasional spash or where the wet is quickly mopped up. I have hard pine floors finished with several coats of poly in my bathroom that works fine. My shower is a porcelain tub.

On the whole, it would be very questionable to put wood in a shower floor.

Disclaimer: Recently, exotic hardwoods have come into widespread use in the US for porches and decks. Ipe, cumaru, and other South American hardwoods can handle repeated wettings without significant deterioration except fading. They often need special tools for cutting and drilling, since they are very hard. But note that, even in deck use, the soaking is probably not as complete and frequent as a shower. These woods can be treated with special oil to prolong life and improve water handling.