Okay, my full take, now that the thread's a few days old.
There's no character development except for Bloodsport and Nanaue. Bloodsport, obviously, gets the arc from "bad" guy to "not as bad guy who tries to help," except, this isn't really much of an arc because we always see him helping (he joins the team to save his daughter!). We mostly get his "I'm bad" from him telling us so often rather than actually seeing it. So, on second thought, Bloodsport doesn't really have much of an arc. Which leaves Nanaue, who does, even if it's simple and trite. He goes from eating machine to eating machine with the power of friendship! He chooses to have friends and act differently towards them. Pretty basic, rather trite, but it's the standout character development of the movie.
The big bad is confused, badly. I had thought that, given the lead up and fight over the evidence of US involvement in the experiments, that the actual Starfish thing was going to turn out to be 'not a bad guy.' That the 'bad guy' was going to be a combination of the different team goals and having to deal with the entirety of the corrupt military of the island without support from Waller (even hinderance from Waller). But, nope, that wraps up neatly in time for the Starfish to go on a rampage with an explicit goal of eating everyone on the island. Why Waller was okay with this, I'm not sure, unless the heroes missed the clear kryptonite of ocean water, because I don't understand why the Starfish was not going to be able to walk/use boats/move to the mainland. Failure to provide limits to the threat. But, I digress. So the Starfish is the bad guy, again, and, as such, pretty underwhelming. The final action scene was pretty bland.
Now, to some other complaints. Harley was awesome with the Presidente. I mean, I loved that sequence -- it showcased her very well throughout, and the shooting after he started saying he was going to feed his enemies to the Starfish was fantastic! Especially the bits where she talked to him as he was dying as if it was a normal conversation. Clearly shows Harley as off her rocker dangerous. But, then, they have her captured. And tortured. And, suddenly, Harley is able to tear through the building of soldiers as if they're toddlers -- she's never in any danger, she's a force of nature, and doesn't even get a scratch tearing through the building. Something she hasn't ever done, nor shown was possible in this movie, as she was captured twice by those same soldiers. It was just weird, and seemed to be put together so that she isn't rescued by Team All Boys. I mean, I get that, but they could have done that scene where Harley stays Harley instead of combat goddess Harley. It was weird. Oh, and Ratcatcher 2 is a bigger threat than the Starfish. Like, wow.
All that said, I actually enjoyed this flick. Probably due to Gunn's humorous influence. I felt it was way better integrated than the first one, and a solid entry into the franchise. I have a pretty low opinion of most of the recent post Snyder Batman DC flicks due to how they seem to really misunderstand the characters, so this one actually rates at the top because it mostly doesn't misunderstand it's characters. Or, maybe, it's using characters I'm not familiar with so it doesn't grate on my like watching Superman get mangled.