Ive got a little direct observation of the SSDI process.
My wife's brother developed serious and debilitating heart problems a few years ago (while I believe his health issues are mostly self inflicted by way of recreational pharmaceuticals is mostly irrelevant). He was a carpet and flooring installer, it can be very hard work. He has approximately 65% heart function, an un-treatable aneurism type thing as well as a few other issues that prevent him from doing the only job he knows. So not capable of any sort of manual labor, not really capable of retraining into a tech field without starting at zero. Even my cynical self concedes he is "disabled". He fought the system for almost 3 years and 4 hearings to get his approval.
My daughter-in-law's sister developed fibromyalgia. she applied for SSDI. When she got in front of the judge for the 1st hearing, along with whatever medical documentation she had she told the judge she should get SSDI because she has the same condition as her mother and her mother gets SSDI. Approved.
My sister has suffered several strokes and is confined to a wheel chair and lives in a nursing home, mostly unable to care for herself, a ward of the state. has to regularly have someone fight just to keep her on whatever medicaid/welfare/ disability coverage she gets. It's not like she is going to get better, ever.
So, my experience tells me that the more actually disabled you are and truly deserving of SSDI the more difficult it is to get it.