SIV POSSIBLE SPOILERI think they focused on the curse of the Souleater and didn't bother to make the other True Runes complex.
In IV, the Rune of Punishment has a HUGE curse; the baerer dies unless he can get it to enter its forgiveness phase (where it stops draining energy when used), which does not happen often.
Also, in SV, the Sun rune DOES have a curse, but it affects more than just the bearer. It drives the bearer power mad and convinces them that they are a god and that all shall bow to their power Mwahahahaha, etc., causing them to unleash the runes power unjustly on any who piss 'em off.
The True Rune of Fire also has a curse, though it is not as obvious. The runes ambitions are so powerful that often it will completley take over the bearer and force him/her to do the Runes will. The other True Runes of the Elements had curses to, but I'm not altogether sure what.
I haven't played II, so I don't know what the curse is.