They can all have power axes, just not at initiative that auto wound on a 6. Getting absolutely mullered by a pie plate after an assault is a problem for nearly every elite unit, but for 300 points for 10 they get:
Initiative AP2 weapons that auto wound on a 6 because reasons.
2 attacks base each
An invulnerable save
Give a LD buff to units around them
The ability to transport the unit and a character if you choose without coughing up the price or force org space of a spartan
They can take thunder hammers for some limited dread protection.
They're a no brainer, auto include unit. They're not like the old days with Sekmet broken, but they don't need the initiative bonus. Their only downside is that they don't get melta bombs so they can be tied up with a dread, but they can at least take some protection against that if they're worried.
Compare that the some of the other legions specials. EC have to pay 410 for 10 walking palatine blades in artificer with spears that downgrade to AP3 if they don't charge or win in one turn, Salamanders pay nearly 300 points for 10 Pyroclasts that have 1 attack base and no AP on their combat weapons, but they have to get within a half dozen inches to fire their guns etc. Phalanx Warders have to pay 5 points per model for a power weapon on their 3+ save and come out at 250 for 10 if you wanted to try to make yellow versions.