These are the typical output of a cycles fire render.
Even with heavy compositing, these fire renders still look “off” and never fit well into a scene.

A big bright gloopy mess.

Why do they always look bad?

Believe it or not Blender actually does a fantastic job at simulating fire and smoke, the main problems are introduced in the shading of the simulation.

1. Simulated flame is treated as glowing volume
   One of the major factors that make fire renders in blender look bad is the way that default shaders deal with the flame emission. Instead of treating it like a gaseous vapour, most fire shaders will simply use the flame factor to determine which part of the volume is emitting light, treating the ‘flame’ component as having a constant density.
2. Lack of visual fidelity
   Because of the default emissive shading, much of the depth perception and resolution of a fire simulation is lost. Not only does this result in an unrealistic shading but much of the simulation data that took so long to generate is not even visible to the camera .
3. No accurate burn control
   A real flame NEVER maintains the same burn density throughout the reaction. The entire combustion process relies on surrounding temperature, oxygen dissipation and fuel concentration along with many other factors. This becomes a real issue for standard shaders, as they are very inflexible with extremely sensitive values.

Additionally these renders take HOURS to setup, simulate, shade, tweak and render (even with quick smoke).