It can feed multiple drives at different speeds as needed. As far if one fails, will the rest fail? I would guess that would depend on how the particular burning software handles it (the few times I have messed with multiple burns at the same time, it was with the intention of getting all good burns). I know I have done 2 at once, and maybe 3 (cannot remember for sure on 3. 4 caused major slowdowns on some burns (they actually burned at very different speeds even though it was the same speed drives and disks).
Keep a couple of things in mind. First, your hard drive has to be fast enough to feed all the burns, and many are not fast enough for multiple burns. What hard drive do you have?
Second, I cannot remember the details (haven’t tried that in a long time, I just use multiple computers), but how you have the drives setup on the ide/sata channels matters (I’m sorry but I cannot remember the details, I just remember that some configurations worked and others didn’t). I actually didn’t get any failed burns, even when burning 4, but in some cases, 16x burns took 30-40 minutes (and burn quality was crap).
Maybe someone that has done it more recently than me can give more info.