Generally speaking, MFJ products are solid engineering. Where they fail is in final assembly and quality control.
Whenever anyone buys an MFJ tuner, it's always a good idea to open it up and inspect every single solder connection, and make sure every screw is tight. Assuming it passes that test, you'll likely never have a problem with their tuners unless you try something stupid with them.
Unfortunately, today's inexperienced and uneducated hams tend to do stupid things with MFJ products.