Unfortunately, "Criminal Minds" simply can not have a truly satisfying, or even decent series finale. The best we can hope for is that nobody gets killed off.
The writers pretty much screwed things up completely by coming up with the whole Chameleon arc, and making that guy the big bad of the final season, and the focus of the final episode(s). He is simply not interesting. At all. Not even his disguise angle is that intetesting or original, since there was already an unsub with that schtick back in season one (Mark Gregory from 1x20 "Charm and Harm"). Even the face removal aspect is nothing new, albeit gruesome: remember 3x6 "About Face"? (I'm surprised they haven't mentioned that... it was Rossi's first episode, after all...)
He is not even threatening, because we know that they're not going to kill off any main characters at this point; if something like that happens (hopefully not), it will happen in the final episode, not in four episodes preceding it. Besides, over the last fifteen years, the BAU has taken out guys like Randall Garner, Frank Breitkopf, George Foyet, Billy Flynn, Ian Doyle, Peter Lewis... and now, sleazy conman with mommy issues is supposed to be that big foe?
That is the problem. The Chameleon arc captures all the worst things about new Criminal Minds: borrowing things from older (and better) episodes, ridiculous story directions and plot devices, plot holes, making it personal somehow, trying so hard to make this unsub the biggest and the baddest and the most difficult to catch even though he clearly isn't... etc.
But it's not just about the poor quality of the Chameleon storyline, or the choice to make him the show's final unsub. It goes a lot deeper than that. The show has gone on for way too long, there is pretty much nothing that we haven't seen by this point, so coming up with the new big bad fifteen years down the road and expecting him to maks for a satisfying multiple-episode arc and a series finale is tricky, to say the least. Of course, they failed completely.
Making this Rossi's pet case doesn't help either, because we have already had three episodes about Rossi being haunted by the case from his past, and they were all more interesting than this.
Lastly, two of the main cast members, that were on the show from the start and for over a decade (Hotch and Morgan), are not here anymore, and the members of the original cast that are still present have been reduced to caricatures of their former selves. Garcia probably got it the worst (able to pull up any information and access any database with few keystrokes while almost having a mental breakdown at the slightest mention of violence, despite having worked for the BAU for almost two decades by now and making presentations on all of the cases, while also being unable to have a phone conversation without sexual innuendos and snarky comebacks), followed by Spencer (pretty much only used for spouting quirky trivia and "character in a peril" plot device), then Emily. Rossi changed the least, but that doesn't change the fact that he had almost nothing to do over the last few seasons, unless you count soap-opera level of personal drama.
Of course, it was Moore's choice to leave, and Gibson stepped way out of line and got fired. But that is the risk of having the show run for over a decade: things change, stuff happens, and you may end up losing some of the cast members that were part of the show from the beginning... and then what?
Luke and Matt are just awful, I'm sorry. The writers' attempts to make them connect with them are laughable, because, once again, we've seen all of that done before, and done better (mostly with Hotch, and, to an extent, with Morgan; neither of which, to make this even more annoying, are on the show anymore). Their mere inclusion in the show felt like a desperate attempt to make up for the loss of original characters, it failed right away, and any further focus on them just disgusts me even more. I have nothing against the actors, and the characters themselves are not bad people by any means, but man... I can't stand them.
Yes, it is not the showrunner's fault that Thomas Gibson and Shemar Moore left. I was OK with Emily replacing Hotch, since I liked her character and was glad to see her back, and if somebody had to be the unit chief, better it be Emily than a new character (or JJ). But there was really no need for Luke Alvez, or Matt Simmons, or Stephen Walker, even with Morgan being gone. Especially not for Walker or Simmons, because the team had always consisted of seven members (including Garcia) anyway. And it could have worked fine with six. I would have been OK with the team consisting of Emily, JJ, Rossi, Reid, Tara and Garcia. If they had to bring in someone new, it shouldn't have been an obvious stand-in for Morgan, let alone two of them.
Tara is OK, I guess, but again, she hasn't had anything to do since season twelve or so.
Not only are those new characters bland and uninteresting, but the team is now so bloated that each character barely gets a few lines per episode. Of course, the unsub taking up fifty percent of the episode doesn't help. I can't stress this enough (even though it is way too late): WE DON'T HAVE TO SEE THE UNSUB FROM THE BEGINNING AND FOLLOW HIM/HER THROUGHOUT THE EPISODE WHILE THE TEAM PLAYS CATCH-UP. Especially not every. Freaking. Episode. I actually think that some of the poorer and admittedly contrived later episodes would have worked much better if we didn't see the unsub from the beginning and the unsub's motivation wasn't clear right away. (13x6 "The Bunker" immediately comes to mind.) At least there would have been an element of mystery and suspense, even if the solution was completely ridiculous.
They had so many opportunities to salvage this show and get it back on the track, instead they just kept digging a bigger hole.