Re: Pressure Points
Regarding the difficulty of application, it seems to me that it depends on the particular intent of the attempt. Several points are related to physical features that, on balance, seem like they would be as possible to hit (with practice) as any of several not-so-easy moves in aikido: the bicep, the neck (some danger here!), the outside of the thigh, the achilles tendon, the knee (also danger), the hip, the groin, and the shoulder joint are all likely candidates if the intent is to distract, disrupt, unbalance, or otherwise fiddle with their ki. Also, you can experiment on yourself as one example to see what works, how it feels, how much force is required, and how small the target is. In fact, this instant, I am poking my own thumb into the twelve o-clock position on my pectoral muscle and am too distracted to continue.
I do agree that the advanced points, the ones based less on strictly physical body structures (nerves, muscles etc)and more on TCM (tradional chinese medicine) meridians, and the advanced techniques using these points, seem very difficult to make use of effectively. That statement is provisional to future experience on the mat with someone who knows more about it than I do, though.