Oh, and I'm not categorizing boys and girls. There is a reason I'm thinking about this today. My oldest daughter with 4 kids (3 girls and 1 boy) called me this morning asking about how to go get an assessment for autism. The boys and girls I'm using are grandchildren. The girl IS easy and the problems she faces may be problems she'll end up having to deal with on her own. The two boys are both very difficult, the youngest being 3, who I got the call about this morning, who is especially difficult.
I'm curious as to what you mean by "difficult", just because I think it may help to have an idea of what kinds of behaviors are you talking about.
For me, I think one of the main difficulties I had as a young child was speech (pronunciation) and language (understanding and using verbal language), but still I didn't get a diagnosis of autism until later in childhood (there is probably a little more to the aspect of opinion of diagnosis in my case that I don't really feel comfortable sharing on a public forum).
I'd say when I was very young (perhaps about 3 or 4), part of the social difficulties I had was probably just "observing" and not joining in with an activity/other children (this is NOT based on what I remember (because I can't remember that time) but based on hearsay/reports from my parents). I was completely quiet in school though from 3-4/5 years old, as in, I did not say anything at all (I did have significant language delay (including receptive language delay until about 7 years old) but when I was 4-5 years old I could speak "real" words and (at least at 5 years old) probably short sentences so my expressive language ability was higher than shown at school).
Anyway, I'm not sure that being that quiet at school would have made me an "easy" child, I guess maybe but at home/outside school I probably was much less of an easy child (e.g. I think I cried quite often). I don't know that I was exactly an "easy" child even at school, but how would I really know since I can't remember during that time?
However, I personally feel the stereotype of girls are that they are usually more "quiet" or less "active" than boys, also I've read theories that girls with autism perhaps show "masking" (copying others to be "normal") behaviors more than boys, or that the ASD diagnostic criteria is based on diagnosis in boys. Also, statistically diagnoses of ASD in boys are higher than in girls, so maybe boys may be "screened" more often for autism or it may be thought of as more likely than in them (just some thoughts - NOT saying this is necessarily true)?
To be honest, I personally do not think that "easier" is necessarily better - if I were a child care worker or preschool/kindergarten/elementary teacher, I would want to know what the child's needs and wants were, and I believe communication is a need. So hypothetically if I was in such a position and a child I was in charge of as a child care worker or teacher doesn't want to/is too scared or hesitant to/unable to speak, to me that's fine, and for now the important thing to me is that the child understands what is being asked, and finds a way to communicate by any means. I mean, maybe a child with less challenging behaviors may extract less energy from those close to him or her, but I'd like to know that the child is able to communicate by whatever means.
I haven't really thought of that maybe girls are less likely to receive a diagnosis of ASD because of less "challenging" behaviors before you mentioned that idea. I think that's interesting; it seems a little similar to the idea of how maybe attention deficit disorder may be less diagnosed in girls especially in those without hyperactivity because they may be less likely to show challenging behaviors.