Whilst there's a correlation of anxious/difficult dogs to rescue dogs, that doesn't mean that's the dog you're going to get.
Re-homing centres are generally excellent and wouldn't put a difficult/anxious dog with an inexperienced (or frail) owner.
The most difficult/anxious dogs will only be re-homed with an existing (calm/trained) dog already in the home, etc.
Generally speaking you know EXACTLY what you're signing up for with a rescue dog. The same can't be said for a puppy, or a dog purchase (where the biggest incentive is a financial one).