Linear Fusion Rifles are not consistently working with Unstoppable Fusion
When selecting the Artifact Perk, Unstoppable Fusion, its possible to make both Fusion Rifles and Linear Fusion Rifles counter unstoppable champions. Linears, however, only counter unstoppable champions when there is a special ammo Fusion Rifle equipped within the same loadout.
If a Linear Fusion Rifle is equipped on its own it will not work against Unstoppable Champions at all.