Kalyna doesn't have the Mr. Fixit perk, which you definitely want for a field mechanic(hacking, traps). I ended up hiring Kalyna later in the game, as her Leadership is useful for my secondary squad(I'm trying to get the blood samples quickly). Otherwise, she's more of a beginner merc, with her Gewehr rifle and Night Vision perks being specially useful early on.
Hitman comes with Hit the Deck and his special perk takes into account his lack of mobility(gain Focused by staying still), so his low AGI is not by itself crippling. High marksmanship and Recoil Control make him pretty good with mobile shot, in my experience, and he also knows how to throw a grenade. Pretty solid merc who's also one of the best trainers, while being much cheaper than his rival Raider.
All in all, I agree that the game is generally pretty balanced insofar as every merc being useful in some way, but the pricing doesn't really reflect each merc's value. Tbh, I think this is OK in principle, but there's a problem with the lack of variety, so you end up getting the same mercs every time. There's Key's Mercs, a pretty good mod I've talked about before, but if you want balance you'll have to dip into the lua files and raise some salaries, as some are way too low and it makes the game a lot easier.
BTW, I think one of the popular mods, probably Tactical AI Project, nerfs Livewire's superpower, so you can only see the enemies' location before the fight starts.