High strength enemies, anything S5 or up, will probably cut down the Krox, since the extra S (anything above 4) is wasted on skinks, and the Krox won't get a save, that and anything S5 or up will likely be expensive and afraid of the Krox strength.... unless youre a marauderer, but sill, high strength enemies will hurt the big guys
Anything S3 and 4 will probably go for the skinks, should cause more damage than trying to hurt the Krox,
Remember skinks now have a 5+6++ they get a better save than the Krox! It's only the Krox toughness that protects them.
The question is should you use them at all?
A unit of 3 Krox and many skinks is about 300 points, so is 6 Krox,
The skrox will have three ranks and a banner, so stubborn and starting with 4 combat res, and the banner being muchly important in 1/3 of games remember, They can shoot, ish, kind of, 10 poisioned shots isn't terrible, but it's something, 2 wounds (almost) on a stand and shoot
The Krox have all the high strength attacks, 18! 7and a half wounds against anything T5 or less, but remeber the extra strenght will be wasted a lot of the time, since high T and 2+ isn't that common, and you are assuming none die, and they will. You only have to take 6 wounds to lose a third of your effectiveness, which isnt that hard to do, But they open chaos armour like it isn't even there, they also aren't a core choice
But you can get 20(something) saurus for that! with 3 probably 2 ranks, banner and a bucket load of attacks,
So, in the end, if you aren't taking saurus as your core, ( for whatever reason) you will (probably) have to take skink cohorts for the banners, and since you want those banners to survive, (MSU skink units with banners is just going to give away VP's) you will probably want bigish units. And the the Krox do add some Killy power, and the bonus leadership.
They also look amazing.

Which is all that really matters.