all sizzle and no steak

/ˈɔːl sˈɪzəl and nˈəʊ stˈeɪk/

/ˈɔːl sˈɪzəl ænd nˈoʊ stˈeɪk/

(of a person or thing) having a flashy or showy exterior, but when examined closely or experienced fully, it fails to meet expectations or deliver on its promises