Well, I have an Arab/Welsh cross. She's good for endurance, but forget about it with something else ... she is hot headed AND stubborn. We have her on a strong Magnesium supplement for both her metabolism AND 'freaky factor'. She doesn't look bad, but sometimes you have to think more of the 'attitude' than just the conformity of the animal. My kids cannot ride her, and she has hundreds and hundreds of miles on her. If she can get away with it, she will; and if she can eat it, she will. And forget about the WHOA.
Worst cross I have ever seen was a Haflinger/TB cross. I suppose that cross could be fine, but this one was not. Everything that could go wrong did. I wish I had a picture because even my husband, who is not a horse person, asked "What is wrong with that horse?". You know it's bad when someone that is blissfully horse ignorant even sees something is amiss.