The dice modifiers issue really begins to emerge in vehicle combat, where all of the above applies, but now there are lots more modifiers, in addition to the pilot and gunnery skill modifiers. For example:
Take a standard NPC pilot 2/+1 gunnery, 2/0 pilot behind the controls of a Hunter. They'll roll 5.5 to hit, and 4.5 to avoid a hit. They get +2 to their attack when stationary, so about a 7.5 to hit on average, while stationary. Which is an unwise thing to do.
Stationary defenders take a -3 penalty to defend, and while cover can give bonuses (+2 for heavy cover), that big -3 penalty is very hard to ameliorate (remember what I said about +3/-3 modifiers breaking the dice mechanic). Moving at Combat Speed will give no modifier or penalty to defense, while moving at Top Speed will usually give a Hunter +2 to defend while rolling. That Hunter can also take an Evasive Action bonus to gain an additional +2 to defense. So now it's 4.5+2 + 2 = 8.5, which will generally dodge most things on average (Max of 11 on 2d6).
Take a more common example of the PC heavy gear pilot with a +2 Agility driving a Cheetah, which has a +2 Maneuver bonus. That is, out of the box, 9.5 to dodge, on average, without taking into consideration cover or movement. The Cheetah is essentially unhittable at its Top Speed (11.5 on average, max of 13 on 2d6).
What happens is that big slow vehicles with lots of armour are sitting ducks, while fast nimble vehicles are much more durable. Armour is far, far less effective than maneuverability, which is fine for a high-mobility mecha action game, until it's not, and no player in their right mind would volunteer to take a big -1 Maneuver brute like a Spitting Cobra or Razorback. They have lots of guns, but they aren't reliable choices.