So there are groups of people who congress intended to protect, but whose mitigating measures takes them out from under the ADA. Thus, a one-eyed truck driver is not disabled because his body mitigates by allowing him to turn his head.
And this was really hard (Toyota). So that's why we get the ADA amendments.
"Undue hardship" is a defense, and therefore must be proven by the employer. And the employer's resources might be a factor (i.e., Wal-Mart has more to work with than a small stand-alone pharmacy).
"Direct threat" is likewise, but it's an individual inquiry, not a broad assessment.
On the other hand, being an addict who isn't currently using (i.e., you've rehabilitated yourself) might count (record of a disability or perceived as having a disability).
The accomodation has to address a problem that the person has related to their disability.
It's an interactive process: the person with the disability comes forward and makes a request. So this means that a person has to disclose their disability. Should you do this in the hiring process? If you do, it's very easy to make an argument about why you weren't selected. Of course, maybe you need an accomodation in the interview process (moving it to someplace accessible, getting an interpreter, etc.).
Next the employer proposes accomodations. It's not up to the employee to pick what they want. The employer can decide how best to accomodate. And then there's back-and-forth.
Lots of litigation here in trucking. Trucking leads to bad backs. Also police officers. Things where you need licenses and stuff.