AoS Kroxigor, Are they usefull?

Exactly

 

Wary fighters won me some game, and it was super satisfying when it kicks in.
Alas, it's happening less and less, for a number of reasons, in order of importance:

1 - thing that fights before the standard combat phase. They annihilate for free your wary skinks, negating their only advantage.
2 - more hordes. you usually don't field skinks in hordes, so even with wary fighter, when you retreat from a mini-horde you will rerely control the obj you was sitting upon
3 - after 5 years, people are aware of the trick. When they need to kill your skinks, they do it.

 

yeah, Knights is the perfect example of old warscroll.
They do just 1 attack with their main weapon, and they trigger 1 MW on a nat 6 to wound.
Compare it to new units, that delivers a greater number of attacks and deals 2 MWs on a 6+ to hit.

the difference is massive, and you must be aware that you cannot count on knights for their combat prowess. They are useful for other reasons. The fact that they can somehow sustain a fight is a bonus.
Skinks cannot fight since the loss of skink chief, and as said before, wary fighter is an AMAZING ability, but with the current increase of rules that let the opponent bypass it, it's becoming weaker,

 

There are ways to position Skinks that will require an opponent 2 turns to take the objective, even if they are large hordes.

 

You can also create rings that prevent an approaching unit without fly from being in range of the objective, ect.

 

In my experience it works very rarely. If the opponent wants the obj and you're guarding with skinks, he will take it.
A great value of skinks lies in wary fighters, but if the skinks are the primary target there will be no chance to save them.
and if you manage to screen the obj even if you are wiped away (encircling it, or similar), thus preventing your opponent to move within reach... at that point is not a merit of the skinks' rule, but it can be done with any kind of screening unit. Even with Knights.

skinks remains a very useful unit, and are perfectly able to steal an objective held by your opponent, but they have wide limitations.

 

It works because you can Warry Fighter and force them to engage twice for the objective. This robs them of the turn and puts you ahead in action economy. If you do this to every objective with Skinks it means your opponent will only be able to claim half (+1) of the objective they engage each turn.

 

This!

 

but the point is also: to drown your opponent in cheap bodies is a tactic that works regardless of the abilities of those bodies.
In this case the ability of skinks is: we are cheap and we can die in spades and be summoned again.
The strong ability is the summon, not the skinks.

 

It is both.

Wary Fighter makes them much more useful than other objective spam units. Skinks preform better both holding/zoning and for capturing objectives. If I charge a unit of 5 Saurus Knights onto an enemy objective that has 6 Ogor gluttons on it I won't be able to score the point with them alone. If I charge 10 Skinks and then Wary Fighter into position on the point then I can steal it with no recourse.

In my experience opponent's will need to spend far more matched play points taking objectives from your Skinks than you have to spend on bringing the Skinks. This is only amplified by the gains Wary Fighter brings to action economy.

 

I'm not saying skinks are not useful. But I'm saying that wary fighter is less strong now than it was before.
You make the example of charging Ogors. Try charging Fyreslayers of a FEC zombie dragon of anything slaanesh, and you'll have dead skinks before they can go away.
The game is evolving, and wary fighter doesn't always work.