I'm not sure what you take to be comfortable - though it is often equated with a more upright geometry and clearance for wider tires. If you look for labels such as 'endurance road', you will often find that along with disc brakes. A good example of such a bike is the Trek Domane (though other manufacturers make similar bikes).

Some of the bicycles marketed as adventure/gravel bikes are also in this category, and can easily be made more road-ish by putting on slick tires.

I personally prefer carbon to steel, though honestly, a comfortable bike can be made out of carbon, aluminum, steel or titanium. In part, it just depends on how much you are willing to spend and your comfort level with different materials (people on this forum all have their own opinions).