You can certainly do some serious indoor training on a spin bike; no one disputes that. The thing is that you're either going to have to find a coach/class like climb14er or do it yourself. Otherwise you're going to get a lot of loud EDM and weird moves like pushups, butt-taps, and hovering that you would never do on an actual bike.
Regarding Peloton, they do have actual cycling-focused training classes on their platform. I did a couple that were led by Christian VanDeVelde that were pretty good.