I also think it depends on what you are looking for.
For most people, if you get a young pup without major issues and work hard at it, if the dog is physically capable and has any drive at all upon which you can expand, you can get a decent obedience/agility/etc. dog. More than adequate for a casual competitor anyway.
I mean Kim came scared, malnourished, and with a very strong "flight" response to all things unexpected (which is probably the reason she was the one pup to survive after being abandoned til rescue was called), and with really no interest at all in people (other than as possible threats or sources of change), etc. and after massive informal socialization and training with a total (mentor-less) rookie with her first dog, much less first competition dog, she was nationally ranked in rally, titled in obe, and is now going for her MACH and often placing in classes of 20, 30+ dogs many of whom are in the top 5 for their breed nationally.
I'm not that special. I'm not that great of a trainer. And I'm not even that lucky. Dogs are just really resilient, in general, and it's amazing what they are capable of given a chance. Even ones with crappy starts to life.
Now if you are looking for the next NAC/NOC, the parameters start to change. You ideally want a dog with abnormally high drives but stable...excellent structure...strong social drive...mental resilience...things that probably won't show up as a very young mystery pup in a reliable way. But honestly when people on a forum ask if a type of dog would be great at agility or whatnot, I generally interpret that as whether the dog would be able and willing to play at agility on a locally competitive level. Because if they were looking for that NAC/NOC dog, if they were serious about it, they probably wouldn't need to be asking the question, or at least not on a general forum like this