I never had problems with chibi (I don't draw them often, but I do like them a lot), but I learned that they are all about proportions indeed!
Originally I drew chibi like 1 (it's 2007,so please cut me some slack =)). You can see that they clearly don't have chibi proportions, so they don't look like they have to. Yeah, the head and eyes are bigger, but the limbs are too long and more "realistic". So they are just... kinda strange kids or something.
Then Morpheus came (2) and I got her chibi look better, because it was a chibi of a child. So yeah, since kids have different proportions, with short limbs, their chibi look more natural too. Not drawing legs under the dress helped a lot too!
Suddenly 10 years passed when I haven't been drawing any chibis, but I went into the contest (3), and I had to do a bit of research for these guys. Definitely found out that chibi are a more simplified style of art and I should work accordingly, also thinking more about small hands and small feet.
The last one are my recent, you can see they became more chubby, and if I draw their legs, those are shorter too.
Definitely no protruding shoulders and other possible parts. I'd probably say the eyes should be right in the middle (or almost) of their head (from hair top to chin). They usually do look better when their butt is slightly bigger or the same size as shoulders. I personally really like Hetalia (original Himaruya art) and Nendoroid chibis, they look the most appealing to me, without very huge eyes or other possible strange and off putting details.
Oh, and answering original question - it's definitely not harder than any other style
Drawing a full realism is so much harder. The proportions and simplification may not be too easy, but it doesn't require that much of work, probably more adaptation to the style if anything. But it should be easier if you already draw in anime or cartoonish style, I think ^^