If you discount the first season, Luther has been consistently solid in what it does and how it does it. The previous (fourth) season was too short, rushed, and unsatisfying, so it was good to see a run of 4 episodes for this season. Within this season it does the stuff you've seen it do before. There is a very weird killer doing very weird killings - Luther is going to get them, but it will be about three and a bit episodes before that happens. Meanwhile Luther is also tied up in some sideplot which he is trying to get ahead of even as it closes in on him. It does this all pretty well in terms of basic mechanics; it enjoys its violence, it enjoys its grim feel, and its characters all slouch and swagger around in a way that implies complexity and darkness. This is how this fifth season goes.
For it to work, it needs you to go with it, so it very much pushes brand recognition. Elba is the main thrust of this and continues to be the reason the show gets made no doubt. He thrusts his hands in his pockets, offers plenty that speaks to a complex, haunted character, and drives into violence with a heart that isn't thee on paper but he makes happen. Luther is far from an interesting character, but Elba is a great presence. The supporting cast are variable and mostly feel disposable, with plenty of them being plot devices. None of it feels particularly grounded in anything, but it ploughs on ahead, providing violence, style, and Elba to carry the viewer along.
Of course this limits it too, but it offers enough strength to just about cover for this, although you do need to ignore the lack of even internal logic, and the heavy reliance on these core values over anything more substantial.