The biggest fault I found in eventing is that most eventers are taught dressage by eventers that have a mindset that shouts out "just get through the dressage part so we can get to the fun jumping portion".
The best eventers will take dressage lessons from a real dressage person, cross country from an eventer and jumping from a real jumper. These people win the most often.
When you see a dressage test in eventing that looks like it could beat an only dressage trained horse then I can guarantee that person was taught by a dressage person. This does not take away from what can be learned by an eventer PROVIDED they don't have a "I really hate dressage but we are stuck with it" thinking.
There is just so much more that can be learned from a dressage focused person, just as you will from a jumper only focused person and an eventer (for cross country portion only) focused person.