Indoors is much hotter - just done 80 minutes on turbo and sweated a good puddle. So you will sweat more.
Is Spinning as good for cycling on the road as cycling on the road - errr no.
Is it good in the winter, yes. Does it make a nice change, yes, especially if the view is good. Is it better than not cycling, yes.
Do some of them not turn the dial up much - yep - lazy feckers. A lot of spin bikes have metal wheels. Friction generates heat. So at the end of the class feel the temperature of the wheel. Last one I did I checked mine and it was too hot to put a flat hand on. The instructor asked me what I was doing. She then checked hers and mine and yelped. Hers was just warm to the touch.
So in short I do a few spin classes across the winter cause I get sick of the garage wall and the turbo. It makes an easy brick with a treadmill at the end on nights when it is dark and grim. It changes the training a bit.
If you are doing sprint tris they may have more benefit than for an IM focus.
All IMHO
M.eface