Under the bench is a common installation location. Steam generators do not really require much cooling or ventilation since the steam generator casing does not get hot during normal operation.
What you need to be watchful for with an under the bench installation is making sure you create an access panel and sufficient space for service, if ever needed. Failing to do so can seriously complicate the repair process which, in turn, can severely inflate the repair time therefore increasing the cost (not to mention making your repair guy unhappy
Read more information about installing a steam generator under the shower bench at www.steamsaunabath.com//catalog/all_about_steam_rooms_location.php#location