Honestly, i think it all depends on preference. as for me, humans are easier to draw than animals because when learning to draw, I mostly focused on drawing humans, and also I enjoyed it more and didn't put much effort into drawing animals. I have also realized that some animals are easier to draw.
I do have a friend who only draw animals because they love it and they draw humans less because it's not something they are into and as for me, I draw humans more because it's just want I enjoy. at the end of it all, I think both animals and humans have an equal amount of difficulty but having a preference makes you put more effort into learning the one you favor more.