You should read more carefully. I clearly defined "accessibility" as:
A) The tech steps (in buildings) that are required to make the unit, so 0, 1 or 2 prerequisite buildings;
B) The ease of use/cost-effectiveness/level of specialization of the unit.
I bumped engineers up to tier 2 precisely because they're overspecialized. Sure, they can win you a game or give you an advantage over your opponent but they can't really save you in a fight and aren't useful for anything else but capturing buildings. They are completely useless in battle, while most of the other base tier units (save perhaps for rocket men and artillery) are generally very well-rounded in combat.
I think you are confusing the regular guard tower that has a machine gun for the advanced guard tower, which I put next to the obelisk at tier 2, where it belongs.
You might have a point about the stealth tank, that it should be at tier 3, but I put it at 2 because I believe it's also quite a versatile unit. Good for scouting, good for harassment, good for precision strikes on buildings, good as a force multiplier because of its high damage output... Its use is rarely viable in 1v1, but it's a great unit in team games and FFA.