Models struggle in 2 main areas. The first is spatial reasoning: often the models make off-by-one errors which they find it hard to recover from (as factories are very sensitive to these mistakes - like in programming). The second is in long-term planning, i.e figuring out what to do strategically, before making tactical subgoals.
The difficulty scales in lab-play generally in proportion to the depth of the production chains. If an item requires several factory segments first, this makes it a lot more challenging. I think this is related to planning though, as the models tend to get down 'into the weeds' of fixing minor issues - rather than coming up with a master plan first.
The main goal of an enemy AI isn't to be the hardest thing in the world, it's to provide an interesting challenge for the player to overcome. It's not necessarily difficult to make a hypercompetent AI in most games, but that also wouldn't make it very interesting to play against. Most games have finite states of logic, just large enough to the point where a human would have trouble finding every solution to it (although humans tend to be very good at pushing on the edges of these states to find ways around them).
Even in games where the amount of state is much higher than usual, you rarely want a super AI; nobody likes playing against an aimbot in an FPS for example.
Factorio is an outlier because unlike regular games, the true condition for a "victory" is almost entirely up to the player. You can make a rocket in non-DLC Factorio (the games victory condition) without building any factory at all beyond the most basic structures for stuff you can't handcraft. It'd be extremely slow, but it's an option. That's why the benchmark for this sort of thing is more efficiency than it is "can this work".
Did you find there are particular types of tasks that the models struggle with? Or does difficulty mostly just scale with the number of items they need to place?