Stealth/Invisibility related UI requires more work. Backstab Assassins are fun to play when they work, but annoying when they don't.
1. Don't hide the action/recovery bars of friendlies when they're invisible. They are crucial to keeping track of what the character is doing, especially since it's hard to see the animations of an invisible character. And it's super annoying when you can't tell if you're about to land the backstab without timing out the invisibility, because you can't tell whether the character is stuck in AI limbo and needs a helpful nudge, or is just waiting on recovery to finish.
2. I know that efficient pathfinding is a hard problem and it will probably never be perfect for all situations. But having an option to draw pathfinding trails on the screen when you give a movement order while paused, would at least allow you to notice when the pathfinding bugs out and fine tune your orders before you unpause. Again this would help you to avoid timing out your invisibilities because the AI bugged out.
3. Show the 2 meter range circle around every character that has both the backstab talent and a ranged weapon equipped, so you know how close you need to sneak to land the backstab, without having to eyeball the distance.
4. The AI script editor needs conditionals for "Self: Stealthed/Invisible", so you can script the character to reserve their special attacks for backstab opportunities.
5. And also some way to set preferred attack range, so you can force the character to close in to backstab with ranged weapons. If you made the range setting adjustable, it would also be helpful for forcing ranged chanters or other ranged characters with offensive aura abilities to close in on aura distance. And allow characters dual wielding one handed guns/implements with different ranges to close in to a range where they can shoot both of their weapons.