If you have a PTSD diagnosis and cannot work, then that is enough to be able to apply for SSDI. Approval is a whole 'nother can of worms and it varies greatly. Your best bet for approval is having a lot of medical documentation. Without documentation, they really have nothing to base your claim on and approval is much less likely. OH, wait.....and there is another requirement that you've worked so much in the last 10 years. If you were full time before you stopped working and worked a number of years, then you probably qualify.
Drawbacks? I honestly have none in my case. Disability isn't a life "sentence" as you can get off of it whenever you are ready to jump back into the working world. I am in the worst category with improvement not likely, but I'm damn well determined to get back to work. What I am actually capable of, that is up in the air at this point, but I'm definitely improving. Since I am at the bottom end of disability income, my medical is also fully paid for (no medicare copays, I just pay a few dollars for each prescription that I fill).
I know that SSDI has enabled me to focus on healing so that I can get better. The "pushing through" attitude isn't always the best. If you're really in a bad place, I know that this can just make someone even worse. I've seen a number of people say that their pride won't allow them to apply, but to my way of thinking, if your pride is what's keeping you from applying, then you don't need it! When I was approved there was less than zero chance of me being able to work anything in the near future. I was constantly in and out of the hospital and bills were piling up as I had no income. Pride? Yeah, this is an INSURANCE situation.....its like saying "I was in a car accident and it was the other uninsured guys fault, and even though I've been faithfully paying my car insurance for years, my pride won't let me make a claim" Yeah, crazy talk. You paid for it and now that you need it, I encourage you to put in an application. I think it could really help your healing in that it can lessen the financial burden and allow you to focus on healing without pushing yourself to work (because that's what we're supposed to be doing as "good Americans".)