My take for what it's worth:
Kevin - in 4 years has had 3 celeb partners who are the wrong side of 40 compared to any previous winners and one clear 'favourite' from the beginning. That he managed to get both Susannah and Kellie to the finals is, to me, a sign of a decent teacher. On a personal basis I do find him a bit manic, but to say he's being favoured is no different to Aljaz, who has had 3 potential contenders and a duffer....
Joanne - first year a real duffer in Scott Mills and did well to get him as far as she did with some clever choreography to hide the worst of his weaknesses. Last year, no celeb - did she grumble and moan and flounce, no she just got on with it and worked hard on the associated bits they gave her and won herself quite a few friends when she did Choreography Corner on ITT, so having a contender this year is her reward for that. To be honest, right at the beginning I didn't necessarily have Ore down as one of the male favourites (thought he'd be behind both Will and Danny and possibly Greg as well).
Karen - until this year has only had one real contender in Mark Wright and even he, at the start, wouldn't necessarily have been picked out as a potential finalist. Other than that she's done her time with the guy from Westlife (or Boyzone...) and Dave Myers and Jeremy Vine getting both of the latter further than some might have thought they would go. This year ok she had a real early favourite in Will, but that didn't turn out so well did it...
So to say they are favoured and always given the best celebs is I think stretching the point a bit....