I've noticed that you're playing v1.0 in the video. There's an official update to v1.07 which fixes a couple of bugs, including the hugely exaggerated building repair costs you mention in the video. Also there are unofficial fixes which address many issues which Westwood somehow overlooked, like an "improved" AI which uses teams of units (it was meant to do that but the the original release contained a mission syntax error that prevented the scripts from being used, and the patch did not fix that).
Regarding the unit limit in the game, there are actually two kinds of limitations. The first is a per House limit which is set in each of the scenario files. The value is in most cases 25 for any House, with the exception for the first few Atreides missions. This affects how many units the player can produce from barracks and factories. It does not distinguish between pre-placed units that you get at the start of the mission, and units built from factories or procured via the Starport. When you have 25 units, the factories will refuse to build more stuff. The Starport ignores this limit, and you can buy more units beyond your production capacity.
The other limit it a global per map limit. It is related to the internal functioning of the game, as each unit present on the map has an individual index from a pre-set array (more on that here if you're interested). There are separate indices reserved for aerial units, ground units, special units (like the Sandworm, Frigate or the Saboteur) and projectiles. This is the "hard" limit of units on the map because the game cannot place more units than it can assign indices to them.
So on later stages you may not have yet reached your 25 unit limit but because there are many opponents that have built lots of units you may not be able to build more units.
As for the AI being keyed to the predesignated start location, it is not entirely correct. The AI is indeed very easy to cheat, because it has a set of priorities for which of the player's buildings to attack first (again you can refer to Nyerguds' editor which allows to edit those priority values). Out of the structures available in the early missions, the Construction Yard and (IIRC) the Refinery have the highest priority, making the impression that the AI is programmed to attack the player's start location. However, the Heavy Factory and the Repair Yard have even higher priority and will become targeted by the AI as soon as they are built.