If you want something more reliable perhaps look at the Firewire 800 cards. The 800 plugs and sockets seem way better designed than the 400 4 or 6 pin variants. Firewire 800 to 4 or 6 pin 400 cables are available.
I think, maybe, because if you use cheap cables and/or don't check them regularly it's possible on the 6 pin plugs for the 12V to short to the signal lines and fry something. Although that cannot happen with 4 pin to 4 pin interconnects the 4 pin sockets are pretty fragile and if you're not careful with 4 pin interconnects you can still fry things if you don't power everything down before plugging / unplugging things.
Another solution would be to still use 6 pin but cut the tracks taking 12V to the sockets.
................I think, maybe, because if you use cheap cables and/or don't check them regularly it's possible on the 6 pin plugs for the 12V to short to the signal lines and fry something.