A pair of nines is literally one of the only hands that has what I would consider a remotely complicated strategy.
Split nines against the following dealer upcards:
Stand against the following dealer upcards:
A dealer hand of 2 - 6 is a stiff hand and the dealer has a good chance of busting. With two nines, you've got a shot at getting two totals of 19, which is a respectable total. Decent expectation in that situation.
If you stood against an 8 or a 9, then you're likely facing a dealer total of 18 or 19. You can only hope for a push in the first situation and a loss in the second, unless you split the cards and get lucky.
If the dealer has a ten or an ace showing, she probably has you beat, so there is no point in throwing more money away on this hand than you have to.
A hard 18 versus a dealer 7 has a decent chance of winning straight up, but there are enough bad things that can happen when hitting a nine that it doesn't make sense to split the nines.