Check out this battle here where a level 1 Kralus and Crystal Jaguar take on level 3 and 4 cards. The ruleset was poison and opportunity. Guess who won?
If you guessed Kralus and the Crystal Jaguar, you would be right. Shields are useless against poison so despite my opponent having a tremendous advantage in the health + shield totals, it wasn't very useful for him.
Not having to worry about taking 2 health damage every round is a huge advantage and add to that the flying ability which helps it miss enemy attacks. Not many level 1 cards offer immunity out there so it is definitely worth investing and getting one.
The biggest disadvantage for Kralus is the fact that it requires 12 mana. So in low mana cap battles, it may not be very useful. But once you go to mana caps above 24, I think they are a must have in my Life splinter lineup.