I originally started with the mk2, but the horde clear is awkward - heavies are too slow. My main concern when switching from the Eviscerator mk2 is that the L1 is incredibly good for dealing with “light” elites and stragglers, and I wanted something like that, but it didn’t really work.
Relic Blade is definitely a “make weapon work button” weapon like the Veteran Power Sword, but the weapon is still serviceable in its base form. Switching between the two forms depending on intensity seems to be important for mastery of the weapon, and it helps that you have guard point during (de)activation.
Main downsides with the weapon is that the heavies and chaining into block/push are really slow. I wouldn’t ever use the heavies and you seem to just be screwed if a poxburster appears.