Fireplaces are fairly straight forward. You need a fireplace, a spfx fire box and adequate ventilation. All the equipment is easily concealed off set.
When you design a set with fire features free standing from the walls of the set you are presented with the challenge of concealing the gear. At a minimum, you will have gas lines running from the flame unit to the gas tanks off set.
Of course it depends on the nature of the set, how you can go about solving it. There are creative ways to conceal gas line within scenic flooring, however a discussion is due with the camera department re: floor dolly shots.
Platforms and steps are great and expensive.
Set Dec sometimes becomes the optimal choice (as long as the dressing makes sense).
Where the flame source is placed in the set can also make a difference. If you know ahead due to good planning that there will be no dolly or action issues (LOL), you have more freedom to design floor features to conceal the pipe.
Sometimes, its a non issue: the shot will be too tight, or the lighting will be to contrasted to see the floor (wink wink)
Its always good to be on set when they are shooting. It's the PD's job to make sure the final scenic picture in the camera is the final scenic picture intended.
Give them a chair at the monitor. Its why you hired them.