A few minutes ago, I had this Type What You Hear exercise. My answer - Bojíme se tohohle stroje - was considered incorrect. Curiously, the correct answer given was... "Bojíme se tohohle stroje." There was no Report or Discuss button. When it came around again, I gave the same answer, which this time was accepted.
All to say, MAYBE something is not working properly on the DL side with this one. (Could have just been a glitch on my end, of course.)
be afraid / be frightened / be scared, I do not see that much difference grammatically that would make one or the other better translation of bojíme. All are to be + something. I consider the latter two stronger, but I am not a native speaker. I also consider those latter two, as they are actually to be + participles, to be a direct translation of "jsme vystrašení z ...". But that is not a reason to reject them.
I'm native AmE, and "frightened of" works fine, though I suspect it may be used less often then other options like! "afraid of" and "scared of," at least in the US. And I agree with VladaFu that there is nothing that makes it a "better" translation than the one shown at the top of the page. But I believe "frightened of" was added as an acceptable alternative for another exercise, so I've added it here.