How can you tell if a basement finishing system is a legitimate one? Is testing needed or, can anyone come up with something that looks pretty on the walls?
I believe that MgO board meets the ICC requirement for flame spread and smoke development. It is currently being used as part of a system used in basement finishing.
Flame smoke spread can be established by UL the most stringent testing which cost 10,000 and then an annual fee and 4 quarterly annual visits to the manufacturers facility. Or you can get ASTMI which is a one time fee and test. If you switch materials or vendors you are still certified.
After years of installing MGO in basements, it does have problems. Is rigid and cracks very easily. It is more difficult to repair, and shows the seams. I would avoid MGO board.
We have a lifetime warranty on our walls and have found MGO suppliers to vary greatly in the quality of the MGO board they sell. Materials matter in the manufacturing process along with quality control and engineering. Sometimes you have to pay more to your vendors in order to achieve a higher standard in manufacturing a quality product.
Seams are not a problem, if your customer knows they will show.
Many of our fellow basement finishing manufacturers continue to use MGO not because it's cheap or easy to ship, but because it works well in the basement. After the hurricane on the east coast the demand for a basement that can handle a flood was the main concern of the people we talked to, and MGO still satisfies that need.