Inquisitor Martyr is.....
It's basic, as an aRPG. I can't really call the combat satisfying throughout. It's rather repetitive, your selection of abilities is fairly fixed. What you can do comes from what weapons you equip and that's like all of 4 possible attacks. You can have an alternate weapon set which brings that up to 8, plus whatever ability you get from your armor. Since you'll use the same kind of weapon and armor throughout the game, only better tiers of it, you by and large only use the same handful of abilities the whole time once you've played the game long enough to unlock the weapons/powers that you feel are cool.
Optimization is an issue in certain parts of the game too.
The real reason to play Inquisitor Martyr IMO is because it's 40k. They clearly love 40k and did their damnedest to infuse the game with that 40k flavor, mostly successfully. There's plenty of grind to be had for new levels, unlocks on the skill tree, gear improvements, customizing your injectors for healing or buffs, and all that. For me the joy is being like "you know what? Heavy bolters are awesome, Imma gonna use a heavy bolter." As a 40k fan getting to pick and choose your loadout is about half the fun. Whether or not that loadout is actually viable is a different story.
And yeah, you can unlock a ton of cosmetics and additional content through just playing the game, which is nice. The treadmill in Martyr is significant.
If Martyr's action was more enjoyable and more varied, I'd have no real problems recommending it. At $20, I can still recommend it. Just don't expect Path of Exile or Diablo levels of aRPG refinement and variety, Martyr's core action is much more simplistic and less flowing than that, and assets get reused a bit too much for my liking which contributes to it feeling repetitive.
If you do get it, I can say with confidence that the most broken class is still Psyker by leaps and bounds.