The chart is for any kind of action: shoot, move, open a door, do nothing. car wars uses speed to vary when in the phases a car gets to move. same concept though.
>>Is everyone subject to the same limitation? Some that aren't. Some even worse?
Yes; yes and yes – some can have 2 actions, others 4, basically from 1-5. Possibly more, or 0 depending on equipment and circumstances.
>>Does the lack of activation hinder play
models with a low number of actions will do less, as the chart is an inverted triangle/bell the concentration of play is in the middle part giving those with many actions outliers to act first or react.
>>Or is it a case where I have a gun nest positioned such that I'll have an advantage shooting at the approaching groups who are in the open but similarly limed?
That gun nest can go on 'overwatch' (an anticipatory action) and stay that way until they fire even if in a phase that they would not normally be able to activate in (because the condition is met that triggers their action cost).