I tried a coaster brake cruiser because of its weatherproof brake, full fenders,chainguard, and simple reliability. Its mild steel may actually be more rustproof than some grades of aluminum, but both generally depend on a coat of paint or oil as well as cleanliness to prevent rust.
It worked very well but limited speed to around 10 mph in foul weather, or slower when you have to battle your way over snowy footprints and the street is too dicey.
I discovered that a repack per season may be enough for the hubs but ruined a BB, so you'd have to clean and repack. However, Ashtabula bb's are under $10. You will have to clean and repack all bearings unless you discover a brand which comes with sealed bearings. I think many cheap mtbs do but not the very lowest ones.
I got rid of it because in dry weather, hard braking was enough to pull the rear axle out of line and cause chain derailments due to lack of tension.
If you go MTB, you will have to find a way to keep it dry or store it outside. If it gets wet and freezes, nothing will work including brakes.
I did a regular 10 mile commute on mine; you just have to refine your concept of how fast to go or how early to set out. As for excercise, you want resistance.
