I'll start off by saying that I believe most special characters, while possibly being difficult for a particular enemy to handle are
rarely worth their points on the battlefield. The MMU concept even provides evidence that most normal characters can be a waste of points (especially if you have a Lord like the Slann mage who can play so many roles at the same time). I find Chakax no exception to this rule. Would I use him? Sure, if I wanted ot make my unit of TG worth another 300+ points to encourage more missile fire and concentration. Of course, I'd only use him if I was playing a game that allowed specials and was doing it just for fun. After all, he seems like one of the more interesting and perhaps usefull SCs in our book.
After taking a look at his rules, I think
not having a Slann makes him even worse, because the TG unit he would be in is no longer unbreakable. Some may argue that if there is no Slann in the first place than he can still make the unit unbreakable but the first sentence's verbage in discussing this rule makes it ultimately clear.
LM Rulebook said:
When leading a unit of Temple Guard joined by a Slann,
The key phrase is
joined by a Slann, meaning that if the TG unit doesn't have a Slann the rule does not apply to you.
The next question that I have would be to figure out why you are paying 300+ points for a model for which you're forfeiting one of its main advantages for taking it.
Some will argue that making the unit unbreakable is a big advantage. I'd say perhaps, but not worth 335 points to get it there, when a Slann provides a Stubborn 9 (which is about as close to unbreakable as you can get).