Skinks just...aren't very good for a knock out drag out fight. They lack in range to be effective, you pretty much need to put them in charge range to shoot. They're bad in close combat. And a 6+ save isn't going to keep them around very long. So if the mission type is kill your enemy, Skinks won't see much table use. And a lot of the lists I've seen are based solely around kill missions. Sure you can field like 30 of them and put them in terrain and give them mystic shield, but face it: you don't wanna move that many models, trying to fit them all in terrain big enough and close enough for the bonus will be rare, and mystic shield is better served on something important and killy.
Now, if you are playing any objective based games they are better. They're good at pulling units away from objectives by using wary fighter and then harassing melee units with a few casualties every turn, leaving the enemy to either abandon an objective while you wary fighter away again, or take casualties as something gets in position to charge them. Or at stretching out to hold a few objectives and getting there faster with their speed as most opponents tend to ignore Skinks because they get ran over in combat. So sprinting them down a table edge gets ignored as a Stegadon rushes at your enemy.
So really it's based on your local meta. I've used them with success to hold objectives and win while my Carnosaur battles dragons and Saurus Guard hold off better units for a turn. But that's really the best they can hope for now. And give them shields so they don't auto die from -1 rend. At least give them that fighting chance.
Kroxigors, while they can be fun, just don't do the job the best. They aren't fast so they're gonna be back with the rest of our non-skink foot troops, of which a group of Saurus or Saurus Guard can punch out more attacks and wounds (especially with the right hero behind them). They deal out moderate damage and 3 of them don't have a very big table footprint. So they can get surrounded easily and killed by superior numbers, which is bad for both a kill everything game or an objective game. They seem to work well as a secondary punch into a unit that's already held down and isn't a swarm. So against something like Bullgors or Stormfiends (similar sized/capable models) they work well if that enemy is already engage with something else (Saurus, Carnosaur, Bastiladon). They can kind of clean out enemy units when they themselves aren't the primary threat due to numbers or model size. If your don't play against a horde type army Kroxigors can work fine, just pair them with another slow unit that can take a charge, then push the Kroxigor into the fray next round.
So both of those units can be good and usable, it just depends on what you play against and how you're playing the game in your area.
