Mines have one simple issue.
They are a one shot tune, which can only be re-tuned in Jap. If your car has come into the country tuned with one in it, it should be a good thing. Buy one second hand and you never know what mods the car it came off had.
With this in mind you can not really expect to do anything major with the car without getting it re-tuned in Jap.
My car came into the country with a blown ECU, I was given a Mines ECU off a wreck. The car started but ran like a dog. Too much fuel and bad cold start maps, I assumed the car it came off was highly modified. I would highly not recommend anyone buy a second hand Mines ECU, but if you have one working fine now, you will be limited to any future mods.
P.S. I have one for a 33 GTR, which I’m not going to see for the above reasons.