The Neo Geo 6 Slot board is as reliable as any of the Neo Geo Boards. The boards are big though, and are a lot more complicated. Just hold a 2 Slot next to a 6 slot and you will see what I mean. If you have a good 6 slot though, it should last you forever as long as you take good care of it.
6-slots supposedly can cause some sort of over-heating or unstably hot problem when hooked up to Super NOVA's/Super Guns I heard, but that's it. A bootleg MVS cartridge like Neo Bomberman will more likely burn out the board.
Looking at SNK's own list of official releases for the MVS, I don't see it. I believe though the story is that it was released, unofficially, in limited quantities, thus a real copy is quite rare, whereas the bootleg ones are more common. Someone please do correct me if I'm wrong.