The UI in this is really good; it was pretty easy to understand what each thing was doing and where things were going. Building belts and things was also quick and easy, which is crucial in an automation game.
Making letters is quite an interesting take on it, though I wonder if more could have been squeezed out of it. E.g. I usually would just break down the stuff I already had if I didn't need that letter, since they produced the right parts for the next letter, or whatever. So it never really got to the point where it felt like I was building a massive efficient factory, which I think is kind of the fantasy of these types of games.
To that point, the time limit is interesting, because it adds a dimension to the automation genre that isn't usually present. I think this has its strengths and weaknesses (one of the core weaknesses being that it disincentivises making neat and efficient factories, and incentivises quick and dirty tricks). I wonder if having a timer run only once you officially start the factory up (having already built everything) would scratch both the itches? I.e. you have to make a factory to create a word/letters efficiently enough that it can create them in X seconds or whatever.