I believe the Solardon is conditionally useful.
I found the bound spell is useful when I'm not bringing a Slann or my Slann is lacking decent ranged damaging spells.
I say the bound spell is very useful if you are running with no Slann, Shadow Slann, Death Slann Beast Slann,
I would consider the bound spell somewhat useful if I was running with a Life Slann, a Metal Slann, a High Magic Slann
without Lore Mastery, Light Slann, or Heaven Slann
I would consider the bound spell redundant if I were using a Fire Slann, WD Slann or Slann with Focus of Mystery, more than one Engine of the Gods, or a two Slann list.
The Initiative boost depends on who you are fighting.
Fighting other Lizardmen? The passive boost is awesome because you are going to strike first in ANY mirror matchup.
Most Ogres, Greenskins, Dwarves, and undead have Initiative 2 so this lets your Saurus Warriors strike at the same time and your Temple Guard and Cold One Cavalry strike first.
Most humans (and a lot of other races) have Initiative 3 so this lets your Temple Guard and Cold One Cavalry.
Against Elves and most Chaos minions the Initiative buff won't matter much at all.
Skinks gain really from the passive buff since they outclass most foes in Initiative already and most of the foes with Initiative of 4 or 5 will butcher Skinks regardless if the Skinks go at the same time of them or not.
I haven't used the Arkadon much. From the two times I used them, found the bonus Swarms are nice but your Swarms lose a lot of utility if you keep them clustered near your Bastiladon the whole game. The S2 hits haven't been useful for me yet but I haven't tried them on low toughness foes yet. I think the key is to match up a Ark of Sotek against low toughness poorly armored targets like human or elf infantry or anyone that's been hexed with Shadow magic.
Regardless of what kind of Basiladon I use, I have decided not to let them get too far from either my army's strongest infantry block or my BSB. Bastiladons seem to die more from failing a leadership test and breaking then they die from being cut down.