From the fancy stats side of things, it seems like Miller's game fits firmly in a bottom pairing setup and only there. His overall possession gives us this impression when generating shots and re and even a stupidly high xGA60 seems to make him look slightly...better? However, in his role of the top four, he more often than not got overrun by players that are far and above his particular skill set. As he was shuffled down the lineup, this was less of a problem, but he still struggled with high pressure situations and occasionally, and bafflingly for him, played the puck passively.
K.Miller is now an unrestricted free agent, and I really don't know if the Bruins will sign him again. He's been in the organization long enough for people to get an opinion of him, he's regularly savaged by the media for being the defensemen in some of the more embarrassing to watch goals against, and has largely not proven himself capable of the minutes he's asked to perform. If they do decide to resign, I really hope it's for cheap.