Fe-Fe3C is not considered an equilibrium phase diagram, because Fe3C is behaving metastable in iron. It is not really a metallurgical reason but a thermochemical reason, in an equilibrium phase diagram, there are no metastable phases. However, to reach equilibrium you need slow cooling or you have to temper the Fe-Fe3C, so that Fe3C can decompose into iron and graphite. Therefore, often the Fe-Fe3C diagram is used, because under normal conditions the metastable Fe3C is stable enough to be relevant in engineering.

Regarding this question: "what are all the factors that are considered for equilibrium?", maybe it's easier to say, what is not considered for equilibrium and the keyword in this case is kinetic.

Treating as equilibrium for diagrams of binary phases of iron - carbon or Iron - iron carbide is of engineering importance only. Reaction kinetics and cooling rates determine the equilibrium conditions.

For all practical cases, no more than 5% is used in an Iron-Carbon alloy. If the carbon percentage were higher, the alloy would become too brittle. On the other hand, cementite or iron carbide (Fe3C) has 6.67% carbon and is metastable, but its decomposition into graphite is so slow that in practice it can be considered a stable phase. Therefore it is more practical to use the metastable diagram: "Iron-Iron Carbide" instead of the stable "Iron-Graphite" diagram. If the two diagrams are compared they are almost identical.