It seems to me that a lot of critics panned Braveheart when it first came out.
Your question was not to me, but I'd be surprised if a Gibson movie ever gets a positive review now...
I'm sure Gibson researches his movies before filming them. I trust - all artistic license aside - he's done a good job with this film.
As we know, the Mayan civilization was in decline by the time the Spaniards arrived in 1520. Nevertheless, though there were still Mayans around at the time, their main cities were abandoned. Despite being an advanced civilization, there's no question they were bloody, but not as bloody and ruthless as the Aztecs.