We'll, I'll try to address all the concerns here as best I can.
As Abwehran Commander said, yes, tanks do still have a use here in this setting - it's just a little different from what we're used to here IRL. And on top of that, it also differs from our own common preconceptions of tanks being
slow machines as well (Leopards Jumping In Snow!). In real life, the turrets
do track pretty fast (forgive all the Allāhu Akbar) and search about quite animately as though their cannon were eyes. Plus, tanks would only turn their turrets faster here in the setting, not to mention the hulls - during WWII, smart commanders would coordinate the driver and the gunner to rotate the hull and turret both in the same direction to track fast moving enemies like the Sherman and Hellcat more quickly. The same can be done here for great effect.
It needs to be kept in mind that the Driver is an independent person with brains, backing up both the Gunner and the Commander after all. The driver will likely be doing everything she can to keep the tank's front armor facing the oncoming enemy, aiding the turret's fast rotation with her hull's own.
Within SARP, as I mentioned before, the role of a tank is different, but still very, very useful. Abwehran already pointed out that the machines would have a stark advantage against power armor in longer range engagements across places like deserts and prairies, but have the tables turned in urban warfare. Even then, within the confines of a city, a tank is incredible backup for the bread and butter Power Armors - it can hang back with its flanks guarded, using its powerful cannon to demolish enemy strong points that PA find and mark, or even move forward and soak up the punishment should PA get in over their heads, covering their retreat.
Clumsy and vulnerable on its own within a city, but still surprisingly useful as backup to PA troops - it's essentially a force multiplier for them.
And though PA are very fast, the weapons I had mounted on the tank here are perfectly up to the task of fighting fast movers - especially in wide open expanses like Arctic Tundra. The commander's remote turret sports dual, hyper velocity gauss autocannons as well as mini missiles. In fact, they're the very same offensive models the Mindy and Daisy use in their calf launchers, fitted into an expanded one for the tank. And though it may seem counter intuitive, the main gun is also perfectly capable of hitting a fast moving, zig-zagging PA.
The reason why that is possible can be attributed to the 125mm HEAT-FG rounds. Those can be set to airburst in proximity to PA, and it too is fired out of a hypervelocity gauss cannon - a projectile traveling that fast is going to be difficult to dodge no matter what when its rounds can turn a near-miss into a powerful explosion and mauling, high speed fragments. Take my word for it - this is a very powerful machine in comparison to previous ones in the setting, and would set a new standard should it be used right.
I admit that mines would damage the hover capabilities of the tank, but, this is a good thing - I'd rather not have the vehicle be completely invulnerable and without some weaknesses. Yes, it has very powerful sensors over most PA, but I'm sure someone will coat a mine in something stealthy enough to get under its nose. Even then though, I had the article specifically state that those units on the underside aren't complete soft spots either - this gives GMs plenty of wiggle room for how they want the vehicle to behave, something I took into consideration when making this.
As a player and GM here on SARP, I absolutely love Power Armor, and even more so after the Iron Man films. But! I also love tanks just as much, and simply don't want them going overlooked here in this setting. That's why this was made.
(Also, the Sabot rounds are meant as the cheap ammo - programmable HEAT-FG is the standard, producing a piercing lance of liquid hot metal AND a big explosion!)