In theory, they seem good - you get the strength and armour of Krox, and most of the time combat units will only be able to attack them. You get the shooting and bonus attacks of the skinks. What's wrong with them in practice?
I don't know if there is anything *wrong* with skrox units as long as you're not trying to use them for something they aren't good at. You probably won't win too many combats with them. But you might be able to get a few wounds onto a character or monster with the Kroxigor until your actual fighty units get a good charge in. I like to throw a single Kroxigor into a Cohort of Skinks just to boost their Leadership up to 7. The more likely reading of that rule is that your opponent gets to choose whether to attack the Kroxigor or the Skinks. As in this conversation: Opponent: "Can I attack the Kroxigor in that unit of Skrox?" Lizardman Player: "The only way you can attack the Kroxigor is if you are in base-to-base with a Skink that is in base-to-base with the Kroxigor" Rather than meaning "if you are in b2b with a skink that is b2b with a Kroxigor, you can only attack the Kroxigor"
What is the unit good at? Well, you've got a few (9 or less, usually 6) very high Strength attacks, and plenty of cheap but pretty poor quality ranks. This isn't a unit that is going to do well at killing ranked infantry because it simply won't be able to deliver the volume of attacks needed to win. So what are limited high strength attacks good at? Killing limited number, high Toughness/high Armour Save models: Cavalry, Monstrous Cavalry, Monstrous Infantry, Monsters, Chariots, etc. Great! Let's go find those to fight. Except... the Kroxigor can be attacked in combat so long as you are in base contact with a Skink. The Kroxigor only have T4 4+, and with their Great Weapons Always Strike Last. All those things I listed above, for the most part, tend to have a high damage output (At least on the turn they charge) - So they can direct all their attacks against the Kroxigor, wipe them out or reduce them to the point that the Krox WS3 makes any surviving attacks pointless because so many miss, and then spend the next turn crushing Skinks until the unit breaks and flees. About the only ideal thing that they are good at fighting is Cavalry who haven't charged and don't get lance/spear bonuses, or flank attacks on Cav/Monstrous Cav/Inf. Magic does also help shift the balance in their favor (that old Lizardmen chestnut...). So whilst they look good on paper, finding a worthwhile match up in practice isn't as easy at is looks.
Skrox have done well for me against chariots and monsters. Despite what the nay-sayers say, the krox seem to hold up well enough to have enough of them swinging back. Actually, having an opponent swing on the WS3 T4 4+ save krox, instead of the WS2 T2 5+ save skink has won me combats. It's not easy to put wounds on krox, and attempts at wounds on a krox is giving up wounds on skinks. The big problem is Predatory Fighter. You get the disadvantage of predatory (must pursue) but don't get the bonus attacks when supporting (support capped at 3 attacks). Skrox are Rad at holding a building though. You lob out javelins, stand and fire with them, and then have to Krox step up and smash face inside the building. -Matt