I teach spin classes and I also cycle outdoors. The two are not equivalent.

In spin class hills are simulated by adding resistance to the bike, which means mashing in a higher gear. On the road I'd be using a smaller gear and higher cadence to go uphill.

Still, spin classes are convenient in that you can get on a bike and not need to worry about cars, weather, or stopping for red lights. I do a lot of interval training on spin bikes because it's harder to do it out on the road.