Purely human enemies is an interesting idea, but the script is a mess and the execution is boring. The special effect of Walker dropping into the ocean at the end is just awful. I miss the Henson animatronics. Chinese and Koreans play some of the Japanese roles in post-Sekigahara Japan, messing up their Japanese accents, etc.
April O'Neil gets virtually nothing done in Japan except to generate dubious romantic tension—perhaps it's just sexual threat—with Casey Jones, as usual, except it's through his lookalike, Whit. She cuts off the bottom of her kimono, is sexualized by turtles for it, and states “I'm allowed; I'm on vacation”, indicating that she is not allowed to expose her flesh where her reputation is policed. The other female character in the movie is an uncharacterized virago; no passing the Bechdel test here.