Hard to see the words when they start stacking. And worse, you start typing a word and it picks a different word from the one you wanted and you have switch contexts. That's the hardest part of the game, not the typing.
I type really fast (120+ wpm) and a big problem I had with this game was making one typo and not realizing it until I'd already typed 3-4 more words. Then I'd have to go back, find the one typo I made, finish that word, then re-type all the other words that the game had ignored my typing in the meantime. This is actually what killed me in the end.
I got caught by this too, but it was fun. It's the first time I have actually done a test that measures my typing speed without becoming bored after two sentences (I did 43 wpm, I don't even know if it is decent or not anyway).
Same, I think if the ship was placed in the center of the playable zone, and the words came radially towards it, that I would be slightly better at it.