Phasma has totally been under utilized, so much so, that I honestly can't remember anything important that she did in The Last Jedi. Which is a real shame, because embracing an extended universe or not, if you want a character to be a key character, they need to be fleshed out and developed in the film. My same criticism applies to the use and development of Snoke in this trilogy and Boba Fett in the original trilogy. Everyone remembers Boba Fett for being a total bad ass, but that's all due to novels and such. In The Empire Strikes Back, all he really did was track Han and Leia and their party to Cloud City, then Vader basically did his job for him. He was even worse in The Return of the Jedi, his death being reduced to an on screen gag.
I can barely talk about how Poe acted like anything but a respectable leader in that film. Dude was acting straight up crazy and his refusal to listen to his superiors put so many people in harms way. That's something you'd expect from a rookie private, not someone already in a position of command, responsibility, and accountability. I'd have been much more okay with that kind of behavior from someone like Finn or Rey, but for an already established leader like Poe, it seems inappropriate.
If the story was better written, the whole trip to the casiono planet wouldn't be necessary. The whole chase, when you look at it differently is kind of like a siege. Yes, they're moving, but it the resistance's options are limited and time is running out, much like a siege. There have been films that work with those limitations and tell compelling stories. In fact, going back to Poe, if the film was written focused on more realistic efforts for the resistance to collaborate and overcome said siege, and he worked with both leadership and every day soldiers to get it done, he would have shined much better and you would be able to get some real, meaningful character development for him. The film kind of proves my point for me on the salt mine planet, which is basically, a second siege in the same film. It still wasn't perfect, but the movie felt much more put together in that area.
Like I said, I could go on and on about how flawed this movie is. I wanted to enjoy it, I really did, because I've been a huge Star Wars fan since I was eight and I immediately fell in love when my mother rented me Return of the Jedi. It's just that as a whole, from writing to execution, this movie is really rough and the only thing that it has going for it is that it's part of the Star Wars franchise. Unfortunately, I don't think that's enough to save it.