I have 3 minis and one Shetland and all have very different personalities. I grew up as a child with the understanding that Shetlands were nasty, pig headed and stubborn and definitely not suited to a beginner or to a child for that matter. I was told they constantly bite, kick and can be very sneaky.
Now as an adult , I couldn't disagree more with what I was told as a kid. I purchased my first Shetland 10 years ago, the day i purchased my first Mini. The mini breeder had the one Shetland on her property that her neighbor had bred, but moved away and asked if she could find a home for him. I had my eye on a particular mini ( who I ended up buying) and next to his paddock was this Shetland ( an unbroken 3yo).
Well as we all know , its impossible to just have the one, so I agreed to purchase him too. If anything I had more problems in regards to nipping ect with my mini than I did with the Shetland. I halter trained him and put him back into the paddock so he could mature a little more. When I brought him back into work, I was shocked at how quiet he really was and the more time I spent with him, the quieter he became. I spent a good year or so , breaking him in with the saddle pad and teaching him to float ect.
Fast forward ten years or so, and he is happily leased out to an 8yo girl as a lead line pony.
I believe all horses are different and have a story to tell , some can be helped to develop and some cannot.