With the Hero Session you still get the benefit of a smaller and more compact camera compared to the Hero 5 and traditional GoPro models, but you lose a few key features like voice control, video stabilization, and the ability to auto-upload to the cloud. Additionally the Hero Session only offers an 8MP camera (as opposed to 10MP on the Hero 5 Session) and 10 frames per second burst mode (as opposed to 30 FPS). For the smaller price tag, you're also sacrificing 4k video, as the Hero Session only records up to 1440P30 video resolution.
Also see: GoPro Camera Alternatives.