The good: The interplay between Knightley and Whishaw. They may or may not get on in real life but their mutual affection was not only believable, it was the best part of the show. I also think the production values quite good, and all the actors did a fine job with the material given them. First-rate acting.
The bad: Knightley is a fantastic, charming actress, and I might even buy her as a badass fighter with the right tools (knives, etc.) but not in the 10 years it apparently took (in story terms) to move her from outcast normie to badass who can disarm, outfight, and casually murder a former SAS operative. No, no, no. And Whishaw might be plausibly of course good with guns and even knives or other ranged weapons, but again, he is no physical force to be reckoned with by anyone who actually fights. One flashback where we see his alleged goodness is undone when we see another circumstance where his victim is...we I won't say but come on. He's either a sociopath or not, but choose one please.
Also bad: Hard to sympathize with murderers and amoral spies who sell themselves to the highest bidder. That's an odd premise, having these as our protagonists, whom we are apparently meant to root for. No love of country, no moral compass (except maybe "children good", which, yes, I suppose that's something.) Knightley's charm and Whishaw's gentleness go a good way toward getting one past this, but not nearly far enough.
A person is shot and manages to continue functioning rather normally, and apparently a few pain pills and a bandage and one can carry on after an abdominal bullet wound. Who knew? I think that happens twice actually.
No one seemed particularly comfortable or competent with firearms (though Knightley was believable with a knife), though everyone holding them was supposed to be very well-trained.
Generally I get the sense this was written by someone who has never really experienced handling guns or being around real violence. But knows quite a lot about writing characters into and out of very implausible situations, filling the cast with minority (is that ok to say?) characters and keeping straight white dudes to an absolute minimum (unless they are dullards, ineffectual, and/or corrupt).
I rate this 3 because I kept watching it compulsively, so they must have done something right. But I felt, alas, stupider and worse of a human being for having watched it. Please give K and B something better next season, I am sure they could do wonders with a good story and more believable writing.