You can apply infernalism to soldiers with PST 33-43 (Spartanism requires PST 44). I don't see any other benefits to infernalism when compared to spartanism. (And below PST 33 you apply kyberos.)
I've always obtained spartanism before infernalism; starting the apocalypse arc (the communion mission) usually starts later than you get a sniff of the syndicate, and it takes a while to finish it. Even if you wait until you get strong psionics before dealing with Syndicate HQ, that's usually earlier than the end of apocalypse arc. If the mod design started from scratch, I suppose those could have been reordered somehow so that the "best" transformation is usually obtained last.
So far I've never bothered to play far enough to finish the zombie arc get proteanism. The arc takes a long while. That transformation doesn't seem very useful. Those that actively save scum don't need it. And those that play ironman have developed good enough tactics so that their soldiers don't die all the time and the main benefit of proteanism is not a major selling point.
