My Mellotron has given me reliable service for many years. I fried my PML card because I made a stupid mistake that I otherwise wouldn't have made if I was clear headed and patient instead of exhausted and rushing things. I and many others have played my Mellotrons live many, many times without failure or any reliability issues. Once you get one and make it right, it will give reliable service for years to come with only regular maintenance. If you are not in the mood to spend time making a Mellotron right and keeping it that way, don't buy one. Even a new Mark VI is going to require attention on a regular basis.
It's their nature.
Frank