I want share my experience with artifacts reforge and which artifacts are useful.

Basics about artifacts

For the range heroes you have to focus on the artifacts with special skill and for the meele heroes withr HP or DPS. Artifacts with two slots gives the most bonus for special skill or DPS and one slot artifacts for HP. So i prefer to focus at this artifacts.

There are many two slot artifacts with different base stats. For all range heroes you want items with a higher special skill. For example:

r/RaidHeroesTotalWar - green artifact
green artifact

This artifact is useful without reforge and with reforge to yellow it gets 200% bonus for special skill. That is the maximum bonus for a level 5 artifact. Some example:

r/RaidHeroesTotalWar - yellow artifact
yellow artifact

Reforge

But there are two slot items with DPS at main stat:

r/RaidHeroesTotalWar - green artifact with DPS as main
green artifact with DPS as main

This artifacts can be reforge to 200% DPS and 200% Special Skill as a red artifact:

r/RaidHeroesTotalWar - red artifact
red artifact

So every two slot item with DPS and special skill and be reforge to a level 200% DPS and 200% Special Skill item. This take a lot yellow and red gems, so first focus on useful artifacts with right stats.