I think it's individual dogs. When I was taking a training course they recommended raisins for treats, then vets said raisins were toxic to dogs. Ditto grapes, and I had a dog who loved grapes.

It's probably similar to people and has a lot to do with gut health and tolerances. My dog Pascal can eat anything, but he has enough drool in his mouth (sign of good health) to dissolve a safety pin (dogs' guts can actually dissolve safety pins).

If you ever come to Maine for a visit, I'll make a vanilla cake and eschew the chocolate! 

 That sounds like a fun trip!

Thank goodness dogs’ stomachs can dissolve so much, because I know they love swallowing things.  I saw a slide show once, presented by a veterinarian, of all the silly things they swallow.  She showed the real X-rays.  I don’t remember it all, but I think at least one dog swallowed a pen or pencil. 

 It is individual dogs, theobromine acts on the like caffeine does on people: some people are fine, some have heart palpitations.