TLDR: Warp Battery good, everything else is bad and needs a serious change/ nerfs to frustration factor.
Class Penances are not “difficult,” in fact, the word to use is frustrating or exhausting. (I want to point out I have done all Psyker Class Penances other than Pick n’ Mix before the get gooders show up.)
No matter how good or bad you are, the class penances are designed to intentionally frustrate you and screw over your team, and if you get them unlocked, you either are incredibly lucky, or got one of the few “easy” penances that can be done as long as you go out of your way to beg your teammates to play a very specific, suboptimal way. We only receive brief glimpses of actually well-designed Penances.
All classes have challenges like this, but since it’s most relevant to me, I’ll focus on Psyker: Psykinetic. Cliffhanger, go out with a bang, and Pick n’ Mix are all designed to annoy your teammates and are realistically impossible to do with even 1 random.
Cliffhanger requires you to launch 7 enemies into the abyss within 20 seconds, the problem is, hordes don’t kindly line up next to precarious cliffs, and those kinds of cliffs are few and far between on any map. The only map with a good spot for launching enemies is at the start of the Chasm Station mission with a small 1-person wide bridge on your left, but the bridge is out of your way, and if a horde spawns that early, it will before that bridge. These parameters mean that unless you are in voice chat with a cooperative team, the bridge will just be passed without a second thought, you’ll be waiting around for a horde to spawn at the bridge and unless you get those teammates to stand on the bridge as bait, the enemy horde is likely never to go across it for you to Force Push them off for the Penance. Not the worst of the 3, but still annoying and not designed with the natural flow of gameplay in mind.
Go out with a Bang is the first “bad” penance, blow up 3 elites with a perils of the warp explosion on Malice or harder. The Perils explosion is actually quite difficult to naturally get without using something like the Surge staff, or utter carelessness, especially when compared to Sienna’s Overheat in VT2, Psykers are taught to not blow up, as it downs you, leaving the team without a player; and to top it all off, the explosion has falloff damage and doesn’t naturally one-shot elites. This leaves the psyker and their hopefully willing team, trying desperately to herd cats (ragers/maulers/and gunners) near each other, then bring them low enough to get 1 shot by a point-blank explosion (with no healthbars to help, just guesswork and hope for no crits), then requires the psyker to manually blow themselves up without getting downed by the aggroed mobs. You are throwing, by playing really badly, intentionally, and this is somehow expressive of one’s skill or teamwork?
Pick n’ Mix is just bad. Brain Burst used to be too good, so they trashed it, now you can get a max of 3 bursts before being at 100% peril, requiring a vent, and on Heresy difficulty only 4 of the 14 specials/elites in the game can be 1 shot, 4 of them need 3 Brain Bursts, and the rest 2 Brain Bursts, so what’s the challenge? Kill 5 elites/specials with Brain Burst in 12 seconds… (this was 10 before lol)
The better Brain Burst talent is required then (if you prefer something else then tough, I guess). Now you just have to get 5 elites to spawn reasonably close to each other, which only happens on certain spots of the map (not guaranteed), or during a horde (also not guaranteed) 
. Good luck hitting those tiny elites with Brain Burst, while those elites are mixed into a horde, or just be lucky forehead and get 5 gunners to spawn next to each other, which is clearly the expectation set by this insane challenge. How skillful!
Somebody I was helping do penances the other day said it took him 6 days to get this penance, and it only happened cause 5 gunners spawned, and the rest of the team were hyper focused on helping him get it, how nice .
The solo a boss challenge is one of the “easy” penances in that it is almost literally impossible to do without all 4 squad members in a voice channel together, if someone crashes and a bot/random take their place, they will mindlessly shoot the monster, ruining the challenge, or the other thing that happens is the AI director notices the psyker alone focusing the boss while your team clears the horde/ambient mobs, and sic’s 2 hounds/muties on you, because clearly you needed the extra frustration.
Although not all is hot garbage, Warp Battery is easy sure, that’s why you get it early in the challenges list, but it still displays a degree of skill in the psyker. Maintain max warp charges for 300 seconds, not 300 seconds straight on damnation without taking melee damage or peril dropping below 50%, just 300 seconds total before the match ends. Simple enough, but not every fledgling psyker is going to figure out how to do that, so those who can, display they’re a cut above, if just a little bit.
I’m not gonna rumor monger and say that the frustration of the penances is a scheme to sell hats or anything, these are clearly here to test the players. It’s just the penances need to feel more natural to the flow of the psyker, maybe a penance having to do with a force staff to sorta nudge newer psykers off of firearms, remove the penances that are just designed counterintuitively to teamwork or the correct way to play like Go out with a Bang, or have the difficulty of the penances lowered when the ability tied to the penance is nerfed, like with Pick n’ Mix.
I like the Penance gear, I want to use the Penance gear, but I also want to feel like I did something deserving of the cosmetic and didn’t just spend a week straight mashing my head against a wall till the stars aligned.
Thanks for reading the whole thing if you did, I appreciate it.