I can only speak from my experience. I did not know that Parkinsons was an automatically approved condition. I understand that they have a few but that may be ALS or something like it where they assign it expedited approval.
I applied on my own and without a lawyer. I applied in November, was approved in January and since they had to back date my application for some reason I got benefits in February. Years later my wife applied based on anxiety and mental health issues. We applied on our own and again without a lawyer. She too was approved and recieved benefits after her waiting period. The key is the paper trail. You have to have a paper trail to document and support the condition which you have. The listings of impairments tells you what symptoms they are looking for. In short you must have what they are looking for to be considered for approval. In my opinion they are not there to deny your benefits so make it easy for them to say yes. Imagine someone who is just a paper pusher. They can't talk to you, so if you don't tell them specifically enough they have to deny your benefits. Many things are subjective (open to interpretation). Make it not so, answer questions completely and clearly. when it asks to describe symptoms don't put I get pain in whatever. Put down I get sharp pain, which runs down my leg and causes me to ---------. Etc. Hope it helps.

I forgot to add that since I did such a wonderful job of getting my wife approved for benefits she left me some 6 months later!