A typical plotline starts off with a group of main characters with a common problem, then builds upon by diverging out from that central story. We are gradually introduced to related characters and problems that are still connected to the main storyline. It's a straightforward but effective formula for develop a story that engages with the audience.
However in the Tunnel, we start off with multiple parallel plotlines. We know of the central one with our 2 main protagonists and the Truth Terrorist. However, the 3 or 4 other plotlines that run parallel to this are executed in what feels haphazard and disjointed. Already frustrating and weary enough from a central plotline that is getting convoluted, viewers are subjected to these plotlines that are presented in a way that intentionally obscures their relevance to the story, and makes viewers question: "Wait a minute now, who are these people, and why are they showing up right now?" Once their stories are unraveled and these plotlinse converges back to the main one, this when we have to backtrack and recall why their relevance matters.
As said above, halfway through the series, and I'm finding myself weary and frustrated with the kind storytelling strategy that the writers have chosen to use.