https://www.hollywoodreporter.com/review/rookie-review-1152468

The Rookie is decent enough, but its main flaw is that the show is built too much around Fillion and his rookie cop character. "The show's voice is tailored around Fillion and his sturdy leading performance and most of the other characters are thin, denying Fillion a worthy foil for back-and-forth banter," says Daniel Fienberg. He adds: "The series tries, with limited success, to be both a star vehicle for Fillion and also an ensemble. It doesn't pay off in the pilot, creating too many characters and too many plotlines for a rhythm to ever get properly established, yet I understand the desire to at least try to establish a full supporting universe given that I watched over 170 episodes of Castle without being able to tell you the name of the two cops who worked with Castle and Beckett from the very first episode on."

ALSO:

There's not much to The Rookie besides the obvious, but familiarity is kind of the point (https://www.indiewire.com/2018/10/the-rookie-review-nathan-fillion-bangs-his-partner-1202010558/)
The Rookie is a long, sad, drawn-out joke about old age (https://deadline.com/2018/10/the-rookie-review-nathan-fillion-abc-alexi-hawley-1202482539/)
Nathan Fillion's performance isn't bad, but the show is full of familiar cop-show types and tropes (https://www.usatoday.com/story/life/tv/2018/10/15/nathan-fillion-rookie-review-abc/1598976002/)
Fillion on signing on for The Rookie without a script: "Why would I say no?" (https://ew.com/tv/2018/09/13/nathan-fillion-the-rookie-preview/)