[Malorn]

Regarding PS1 interiors, I think we forget how vehicle zerged the courtyards of bases were. I remember many a time I was camped into the base or tower, unable to get out without tanks and hovering reavers shooting into doorways the moment they were opened. It's just as bad of a situation as PS2, with two key differences being that in PS1 you didn't need to control the exterior in order to have control of the base, and the spawn room was near the control point. In PS2 you are required to leave the confines of the interior and move out to exterior areas (like generators) and secure them. There is no safe haven for defenders against vehicles. Once you lose control of the exterior the battle is over.

So when I see "undefendable" I believe it is because:
1) There are objectives in the exterior of the facility which can be influenced by the presence of vehicles.
2) The defender spawn is far from the capture point, meaning the defenders need to cross no man's land to get there while the attackers can park an AMS right on top of the point.

To illustrate, see bio labs - they are the exception and actually are defensible because the two things I listed above are not true. The spawn is very close to the points, and the dome protects the area from vehicle influence. Bio labs are actually very close to PS1-style facilities and have the same rocket-spam on the landing pads as you saw in PS1 facility doors after the courtyard got overrun.