Good observation Denim, Belial was modeled originally after Shai'Tan though we took his special interactions with Lews Therin and gave them to Sisyphus whose background gave more legitimacy to the events.
Belial is also our "intro" Old One, his powers are meant to allow you to recover from mistakes and poor planning. If you play as him originally, as you probably should, you won't see them as a crutch but as a fundamental element that is lost when transitioning to a different Old One. For instance, his ability to create Infiltration anywhere on the map means that even if you haven't had the foresight to plant a spy in a nation that may soon go to war with your allies you can quickly rectify that. In addition to that, his power over random events gives you the foresight to be able to capitalize on what otherwise would require you to well prepared for.
On top of that, he has abilities that hasten movement, cause effects to linger over multiple turns, and your more generic hero buffs. All in all, he reinforces the general mechanics of the game without distorting them. It may seem like that makes him "boring" but on the contrary his abilties make pursuing any path feasible and can lead to really interesting and unexpected situations. Also when he rises..... his power over random events is very, very fun.
Sisyphus represents the Fallen Man, who wanted to make the world perfect but in doing so lost his humanity. He has a lot of interesting powers revolving around agents, going so far as to create a council that a certain number of agents can join. He also wishes to bring the Chosen One to his side, and a massive arch revolves around that - for both the Chosen One and Sisyphus himself. His powers eventually allow him to create zones of "perfect tranquility" where his reign is absolute, and he can personally teach powerful ancient rituals to his agents.
Sisyphus was my favorite choice when we finished him, and remained so for awhile - but at this point I've played out his plot so much that I rarely use him for testing. Still, his ability to pacify the world combined with powerful heroes generates this incredible amount of tension on the sub-level below the military. As armies stand idly by and the world appears motionless, the heroes of the world are desperately trying to undo what you've done.
Kath is fun, and will always be fun - I never tire of raising massive orc hordes and burning places done. He is absolutely terrible on our Guilds and Trading map right now, it's almost sad seeing his orcs just sit waiting at the shore for boats that never come. Not sure if we need to adjust him, or if we simply allow that maybe some worlds just aren't meant to be ruled by some Old Ones.