There's a reasonable chance that the designs simply are not buildable by the computer faction, especially if you're in the habit of stacking as many levels as you can on the Hyperion Shrinker or attempting to obtain as many of the component capacity cost reductions and hull capacity bonuses out of the tech tree that you can, or consistently choose the hull capacity bonus in the Design Revolution event regardless of the faction's ideological leaning. You can see this yourself if you've had a game where you had a good run and got a lot of the capacity bonus events or had an especially high level Hyperion Shrinker; the next game you play, you'll find that you're not able to build some of the ships you designed at roughly the same point in the previous game, and there's a possibility you'll never be able to build them, simply because the hull capacity required by the components is beyond your reach. You can easily see this with custom factions that use the same ship sets but which each start with different capacity bonuses; if you start a game with a custom faction with +20% hull capacity which uses, say, the Terran ship set, and  design an exploration ship on the first turn which uses up all of the available hull capacity, and then you start another game with another faction using the Terran ship set but which doesn't have the same capacity bonus, you won't be able to use the design you created with the other faction until your hull capacity bonus reaches the same level that the faction which created the design had at the time that the design was created (in this case, +20% hull capacity).

The AI should NOT use the players designs. A player knowing this can purposely make terrible designs and hope the AI picks some of them. Now if the AI only uses the designs that the player actually builds then that helps though it becomes quite lame and boring as it at that point is just playing as China pretty much. What we need is variety in designs, but if coding the AI to intelligently design ships is too much then I guess all this is a start....

The AI generally picks the most powerful design it has available for a given role, so it's unlikely to build purposely terrible designs. Plus, just how desperate an exploiter do you need to be to do this? You can just as easily edit the xml to make the AI build crappy ships if you're that dedicated to cheating.

I have, but I wonder why they'd need my sensorships.  From what I understand, they don't even have FOW on godlike.

I've seen AI constructors with 3 constructor modules and even have seen their ships with my initials in their names.  (I sometimes add my initials to designs for specialty ships).

I think it's pretty cool to see my own ship designs opposing me.  As Joeball123 mentioned, I haven't seen the enhanced capacity ones, but I guess the AI still needs to learn how to better position buildings for adjacency effect.

Actually the AI should only use those player design ships that the player made for just that race.

The AI should use players design which are effective. In other words if an AI loses X nr of battles against certain classes then it should incorporate those in his fleets.