I've walked from jobs I thought were too roten or risky for the crew I was with. One problem being a sub is you do not know your ground crew well.
The other day I showed for a big spreading Siberian elm, the contractor did not have the power dropped. I could not guarentee having it down in one day. He willingly paid me for 2 hours. (thanks again)
It's one of those that would be faster with a bucket anyways, due to the spread on it and proximity to the house.
If I'm bidding on nasty trees, I will put a dollar figure that will guarentee me a proffit. Which usually will exclude me from the job Though it has "backfired" on me a few times over the years. It's allways been that when talking to the client I sound the most knowledgable and least cavalier.
I know some guys who have had to scafold a tree and slam the rotten stem out in small chunks.