Unstable client/launcher that is REQUIRED to launch steam bought games, manipulative moderators, abusive practices towards developers, abusive practices towards consumers, ongoing litigation with not only hundreds of violated users but also game studios and even federal law, illegal gambling, lack of a perpetual license. . . I could go on but i think I've made my point.
Steam may be the most well known platform but in recent years it has massively fallen out of favor. Most people now-a-days prefer GoG over Steam but if you have to choose between fully supporting Steam or Itch.io then i would highly recommend itch. It's where I and many others will be.