For me, Orianna's difficulty lies in positioning her ball. Casting QWE while running with your ball behind your opponents can be very tricky. Plus there's the usual mana issues, and when people get to know her, they will avoid the ball like madmen, so landing a good R can be a pain.
Cass is difficult probably because she looks OP, but isn't really; knowing when to engage and when not to might be harder than with normal champions. Other than that, she's got mobility, dot and overall good ratios, plus an escape/chasing R and a very good passive.
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