In my opinion, Kai Leng would have been interesting if he had been a silent character who is professional in his behavior but I may be bias because I feel his dialogue made him seem terribly generic. Kai Leng could have been a silent character who would be genuinely intimidating in gameplay and who would not need to have other characters act unlike themselves to make himself seem a threat. Kai Leng had plenty of potential and I am still of the opinion that the execution of his character was where his potential were ruined.
"Good. You opened this message. This isn't actually asari command. They're busy tending to what's left of their planet. So you survived our fight on Thessia. You're not as weak as I thought. But never forget your best wasn't good enough to stop me. Now an entire planet is dying because you lacked the strength to win. The legend of Shepard needs to be re-written. I hope I'm there for the last chapter. It ends with your death. -KL"
This email is likely intended to serve as a boost of Shepard's supposed feelings of failure and as a way of trying to make the player become personally invested in Kai Leng's defeat as a way of making him pay but to me, it makes Kai Leng seem childish and worse, generic childish. His taunt seems that of a character whose defining trait is that they believe themselves to be superior to their foe when nothing but their ego indicates despite their foe supposedly being concerned that this might be the case.
Kai Leng seems to percieve himself as a personal match for Shepard in terms of combat skill and the game itself seem try to make this the case but this is laughable because the section where this needs to be shown is where he falls so hard: gameplay.
On Thessia, I was an Infiltrator who was unharmed because my sniper rifle calmly dealt with him before he even came halfway close to me. If he'd been a credible threat, he might have taken this into account and altered tactics instead of repeatedly running into the sight of a sniper. It made him look like an awful assassin. A smart tactic might have been to force my character into cover by the way of the gunship laying down fire and serving as a distraction while he took position at a vantage point where he could try to take my character out with a sniper rifle of his own.
Prior to the battle itself, both Kai Leng and the Illusive Man acts very questionable considering their supposed brilliance: Kai Leng casually walks towards Shepard with the gunship blaring a light in the background thereby revealing all of the cards up his sleeve. Vendetta's warning reveals the presence of indoctrinated people, yes, but not who and how many. The Illusive Man could have sent a mook to be the messenger while Kai Leng remained an unseen presence who would have the element of surprise if the situation called for combat.
On Cronos, I was more concerned with keeping track of his Phantom mooks than I was with him. I could never percieve him as a genuine threat because he was never a genuine threat.
Summation: Kai Leng was supposedly recruited by the Illusive Man with a primary reason being his powess in combat. Kai Leng is supposed to have become the preferred operative and assassin of the Illusive Man for over a decade . Kai Leng is supposed to be methodical, ruthless and discreet. Kai Leng, however, is less threatening than his mooks. Kai Leng is unprioritised in terms of danger by his own mooks.