I appreciate and somewhat agree with the idea of what you’re trying to say. But as a main psyker since release, likewise on T5+, it’s plenty obvious to me that there is a problem with BB.
It’s not that it can’t be useful, some builds can make great use out of it. Mostly its issues come from the game’s meta and psyker weapons. The meta is all about massive numbers of enemies, and needing to take them down with abundant AoE, CC and dmg asap. But BB is slow, not particularly powerful, costs a ton of peril, and only hits a single target every 2s without buffs.
Void & Surge do far more dmg for less peril than BB, and have AoE / cleave (well at least Void does). Trauma only loses against biggies but its massive CC and AoE more than makes up for it. That leaves purge and most of the guns, but even with them assail is usually a far better pick for dmg, being fantastic for groups of anything but oggies while adding ridiculous ammo efficiency for gunpsykers. The only thing assail can’t handle are oggies, but even if your team fails in that, any experienced psyker can easily use their melee for that job. And if the oggy groups are too large for Deimos, DSIV or knife? Yeah, BB isn’t going to do any better there.
Kinetic Resonance and EP help a LOT yes, but at what cost? EP exclusively buffs BB and nothing else. Warp Siphon buffs everything and by a lot, but barely matters for BB aside from maybe a few breakpoints. Disrupt Destiny meanwhile adds a huge bonus to all of your finesse hits, yet does nothing for BB at all. Even with Kinetic Flayer you’re going to have to weigh between burning an ult early for a faster BB vs. saving the ult and spamming your staff or whatever else instead (which, overwhelmingly, would be the better option).
So if you build into BB, you will weaken your build in everything else. Yet without those talents BB is so slow and weak that it’s rarely worth using at all, outside of flavour to mix up the game and do anything else rather than just spam your primary / melee. Meanwhile smite and assail need no extra talents at all to be ridiculously valuable, and both of them address the game’s meta exactly either way. On top of that, Warp Siphon is fantastic for both of them, and DD makes assail just crazy (assail also makes stacking DD or WS super easy).
To address some of your specific points:
Smite isn’t at all as vulnerable as you say since you can freely dodge & slide while charging & channeling it. Sure you can do that with everything including BB, but BB doesn’t near-instantly disable everything in front of you.
Assail and Vent address very different needs. Assail brings AoE burst DPS, mixes fantastically between your melee in moderate doses, gives you ranged sniping, and an easy & fast way to ramp up peril for Warp Rider and Vent’s Creeping Flames. It’s not a great match for something like Void outside of the melee thing, but works great with everything else. But assail really isn’t anywhere near enough for those large horde spawns that are everywhere on T5+, and spending it there outside of emergencies just makes sure you won’t have your ammo when you need it for the special+ groups instead. Vent then besides well, venting, brings its massive through-the-map horde clear, adds to Perilous Combustion & Soulblaze in general for some insane SB DoT stacks, and of course CC. Staves depend but usually they’re your focused anti-elite tool.
So with Trauma, assail deals with groups of specials+ or ranged much faster than Trauma would, Vent burn hordes & brings instant CC, and Trauma deals with heavy CC & AoE against large groups / oggies / when assail is on CD. With Surge assail brings more peril efficient instant AoE, Vent again adds to that with horde clear, and Surge deals with heavies or mixed horde backup. With Purge, Assail brings far superior burst and range and is invaluable against specials+, purge brings CC and DoT, and Vent again helps with hordes but now also massively buffs that DoT.
There are so many things I’d like to add but this is already too long.
My point is that BB isn’t useless, there absolutely are ways to build into it and times where it can shine. But it’s just not as frequently useful as the other blitzes and not worth the sacrifice of investing into. I’m thankful to Fatshark for their latest buffs (and fixes, like finally stacking KR & EP) which have definitely improved it a lot, but at the end of the day the problem is still the same. This is a horde shooter. BB doesn’t address that, nor is it strong enough even against the few big enemies to be worth its cast time and cost in general.
Basically, to make BB work I need to make the entire build around BB specifically and think everything else around that, leaving me with few options if I want something actually powerful. But to make smite or assail work? I can just pick up any random idea I like, and at least one of them will fit in there just fine. … Well, less assail and more smite tbh. I can’t stand losing Psykinetic’s Aura, nvm being forced to pick the useless complete waste of a talent that is Mind in Motion. 