Certainly this product is realistic. Any real person who comes into contact with lava will be severely burned and unless removed from the lava immediately will almost certainly die. However consider this. No one on this planet, if represented in a d20 system game, would exceed 5th level. Pathfinder has levels going up to 20th. By the time you're 20th level you are basically a demigod.
Let's say John Smith has "rolled" an 18 for Con and then puts his racial ability bonus into Con, resulting in a 20 Con in total. When turns 18 he enlists in the military and becomes a warrior (or maybe an enforcer, for the purposes of this exercise it doesn't matter). Let us assume that he "rolls" a 10 for hit points. Remember that hit points don't just represent physical resilience but other factors as well. He has a long and successful career attaining four more levels, rolling 10 for hit points each time and using his favored class bonus to attain more hit points. He even takes toughness. He has gone up an age category, but also put his ability point from leveling into Con so he still has 20. At this point John Smith has 85 hit points. He is not only extremely tough, but lucky, and skilled in avoiding damage to a legendary degree, probably the best in history. However one day he has a spot of bad luck and falls into a volcano.
He'll be dead in no more than thirty-five seconds.
So how are the Pathfinder rules for lava not realistic again? High level characters who might, might, survive immersion in lava aren't realistic anyways. The kind of mythic characters they represent are like Hercules and Cu Cuchulain who do incredibly unrealistic stuff in the first place. Could Hercules survive a bath in lava, if only a momentary one? There is really no way to say because he is a figure of myth, which tends to change depending on who tells it. Guess it depends on the story. So if you want people who are immersed in lava to die instantly in your campaigns to die instantly that's you're choice and that's OK. I on the other hand see no problem with a 20th level fighter being able to fall into lava and survive, albeit likely very heavily wounded.
