As knifer you dispatch multiple enemies per turn. There is no time to wait for poisons to kick in.
There's no time to wait for poisons to kick in on any build or in any pressing situation.
The fact of the matter is that in Underrail, it's you vs. a gang of people or creatures who are in your way. Immediate, absolute, and overwhelming force is the king. Subtlety is for losers, unless it's subtlety for the purpose of deploying immediate, absolute, and overwhelming force the moment enemies are aware of you. Once enemies are aware of you, you're essentially locked into combat on that screen for all eternity, even if you leave; it used to be different, but people exploited zoning in and out, so away that went.
Even if you could waste a lot of time shaking your enemies and re-engaging, doing ninja hit-and-runs as it were and waiting for poisons to take effect, it'd be a huge waste of your real-life time compared to mowing them down with an AR. EVEN IF you had the patience for THAT, there's the further fact that it's better to waste an enemy right away before he injects his Krokodil and stimpak, which not only nullifies some of the damage you may just have caused, but also deprives you of loot, since the drugs are consumed.
Here's the reality: When you hit someone, you want them to die that turn, ideally—and if not, just as soon as possible. Some eel piss that takes seven turns to stop someone's heart (or whatever) is useless. Cool concept, but wrong game.
I did my previous playthrough as a psyker and it was pretty much smooth sailing. Now I made a stealth crossbowman and it feels hell of a lot harder. Is it normal? Or am I missing some essential tactic one's supposed to use when sneaking around with a crossbow?
No, that is 100% normal—but believe it or not, Crossbows used to be even weaker than it is now. Just off the top of my head, the to-hit was garbage even at medium range when surrounded by spotlights outputting more lumens than the Sun, and they were (and almost certainly still are) entirely useless against anything with heavy armor.
Styg personally responded to me that "Crossbows need some love" some years back, and they eventually received some, but none of my amazing ideas (such as an additional dedicated scope slot, which would also be entirely in line with real-life crossbow hardpoints) were implemented, so going the Crossbows route is still hardmode.
This is related to poisons also, by the way, because in theory poisoned crossbow bolts are the crossbowman's answer to heavily-armored tin cans. The thing is, you have to do more than 0 damage, or the poison doesn't take effect. Therefore, poisoned crossbow bolts are essentially useless.