Pitbull > Nod-Mods? - Yes. Definetly. At least they cost 10 less. You can win against a pitbull only in a 2v1 scenario. Gotta live with that.
Predator > Scorpion? - While the predator has more HP and deals more damage, the scorpion is faster and has a higher attackspeed. While a scorpion will loose a 1v1, the predator is slow and can be outmaneuvered. Nod has historically (other C&C games) always had weaker units, but they were faster. So, again, scorpion will loose a direct fight, but it has other advantages.
Sandstorm >> Stealthtank? - I hope you send the stank in, fire the rockets and flee. Just sending it in is not good. Usually 3 volleys are enough, so 1-2 stanks are enough. You said the sandstorm killed 3 scorpions without help? I don't believe that. The 3rd one should definitely destroy the sandstorm. I recommend sending infantry at it instead. Or if you only have vehicles, try to approach the sandstorm from 2 different sides. The sandstorm can only attack in a cone, so if you let 1 space between your scorpions, it can't deal damage to both.
But should the sandstorm recieve a damage nerf? Yes actually!
Kodiak >> Inferno? - Dude, you can't compare these 2. You can compare Kodiak with basilisk, yes, but not inferno. They have different roles. Is Kodiak strong? Ohhh yes it is. I am also requesting a nerf, especially in damage to infantry and buildings. The baserush strategy is stupid.
Jackson >> Roxanna? - The 15% increased damage taken seems bad, but you have 15% higher attackspeed than Jackson, so it evens out. With Jackson you can boost whenever you want, with Roxanna you should time the boost: boost a unit once it has been hit by the enemy to reduce damage taken from the shot. This is very important when fighting hit/run units. For example, a widowmaker will survive the volley of an orca and can then take the orca out. But if you boost the widowmaker too early, the first volley may already destroy it because it takes bonus damage.
TL;DR:
Pitbull>Nod-Mods: yes, Predator>Scorpion: in 1v1 yes else no, Sandstorm>>Stank: no, Kodiak>Inferno: not compareable but Kodiak>Basilisk, Jackson>>Roxanna: no.