I don't like the ending very much. As others said, it seems as if the author had lost interest in the series. I read that it was an 80-books series at the beginning, then reduced t0 72, then to 64 and in the end to 54. It seems like if KA had tried to finish the series as soon as possible.
It's too sofly for me. I would have expected an open war, something like "Indipendence Day" with the arrival of Andalite fleet. And Rachel's death...mmm...I don't understand it. As Oceanspray above said :
The animorphs are clearly superheroes; if she wanted realism there would have been casualties a lot sooner.
That's all. It seems as if the author was obliged to obey to a strange rule (and it's not clear where it came out and why) which states : "A main character must die. If not, it's rubbish or book for babies". Well, I think that there are other thing that makes these book childish. It would have been better if only one animorph had survived, then. It would have more sense : all survived or almost all dead. Happy end it's not a crime, even for a war book.
And the way the Auxiliary Animorphs are wiped out from the story...mmm....it's nonsense. No credible reason for this. And the reason for Rachel's death is blatantly weak, imho.
It's as if she had not wanted to write about a post-war future for them.
I admit that, taken out of context, the ending is good, well-written, also intriguing for some aspects, e.g. Arbron and Taxxon rebellion are really exciting.
But, ultimately, it's pretty disappointing, compared to the whole series. It looks very forced, hurried, and leaves a lot of unresolved issues.