Because of how powerful class is - none even dare to discuss it. Any other class seems meaningful (even Ogryn) and that is why people want to balance every other class around psykin.
Why psykin so awesome? It comes in standard pack of two, some would call it all-in-one: psykin host and tzeentch deamon. The power of warp (presumably deamon) allows it to channel unlimited energy from immaterium.
Why is it so fun to play? Because while other classes have to shoot endless hordes with their weapons or just counter zerg hordes with cleaving weapons and brightly engineered power weapons - almighty psykin can use unlimited instruments of devastation.
Always busy: Efficient psykin always balancing peril level. No other class requires you to constantly positioning and quelling. That is most tacticool gameplay.
Puny zealot using flamethrower and psykin can bring more efficient version - purgation staff. Poor zealot will never see enemies under his kneels, while almighty psykin can demonstrate his staff to all enemies around, they will also can demonstrate their weapons in psykin face - that is fair and fancy trade.
While ogryn have slab shield, which is actually limiting his line of sight, psykin can simply bring force sword and consume incoming fire with warp energy. That makes him attacker, apex predator, not puny coward ogryn-like. The only big downside of force sword is psykins dedication. Special attack of force sword can deliver warp energy (presumably deamonic source) into the single enemy he attack. Psykin can oneshot enemy with such attack, while ogryns slab shield special just makes him coward behind the bunker door.
Veterans special ability boosting damage and helps him to easily pick specific enemies, player dedicated. Psykin is above such weakness. Why you have to wait until ability recharges, when you can simply boost weapons with deamonic powers. All you need to do is just collect some warp charges. Constant 16-24% boost to weapons damage. Not much efforts required. Even more to it: you can delegate warp charges collection to your puny allies with “psychic communion” feat, so they will do this job for you.
Ever seen those fancy veteran grenades? Those are limited by 4 and with some luck he can respawn 1 per minute (probably Khorn worshiper). Great psykin have dedicated tools for such job: Voidstrike staff and Trauma staff. As bonus - warpfireballs as light attack. Tons of AoE damage, crazy amount of stagger and also visible impact on the battlefield, unlike puny grenade from some weakling like veteran.
Some people believe that short knife from Catachan regiments can do alot of harm? Well, psykin can use it too, but why limit yourself with some puny knife? Psykin have exclusive access to Maccabian Duelling Sword. Best in line, dedicated fast and deadly attacks. And of course with exclusive blessings, such as riposte. For constant crit. chance boost on dodge.
Amount of exclusive equipment can only be compared with Ogryn’s pull, but we don’t have to compare godlike class with simpleton abhuman.
Blitz action. Brainburst. This is just another league.
Other classes have limited pull of charges, while Psykin can use it whenever he wish and limited only by current peril level. Also you can use this action on the brink of critical peril level without risk of self warp explosion. Precast of BB also allows psykin to kill incoming threats. Also feat Cerebral laceration can boost damage from non-warp energy sources. Which makes Psykin ultimate support.
But there is more: while other classes gain some benefits from new blessing transfer mechanics, devastating weapons of psykin now shine like never before.
That is why everyone so jealous and that is just another reason to nerf psykin even further.
