Not that great.
Against a computer with infinite money? Might as well be. The computer makes no mistakes in guessing letters, and units end up being the same strength in the end, when levels are matched up it's a matter of money and how fast you can pump out units. There's no strategic options, and I think you miscatagorized several things, or at least disputable. Oil Change isn't nessessarily Transportation as it is Automotive. And this is just a personal opinion, but i believe you should choose a difficulty for words, like game difficulty, and then word difficulty.
Aside from that, I think everything else about the game is okay