I don't know about anyone else, but I'm a former Master Tech that worked at a Toyota dealer and various independent shops. I dealt with plenty of JDM engines and transmissions at one time.
My comments are based on my experience, and those of friends and former colleagues who are still in the game.
Just like a used vehicle, they are a gamble. However, if a used vehicle is running and driving when you go to buy it, you have a much better idea of it's condition than you do of just an engine or trans you might buy, but won't know how or if it runs/works until after going to the trouble of installing it/them in a vehicle.
A JDM engine you can remove a valve cover and the oil pan (if the seller allows you to) to get some idea of the engines condition. A trans is bit more difficult to ascertain it's condition without a complete teardown.
The point is they are gamble. Do your due diligence.