I am a native Spanish speaker and probably the best right now, to answer this question. They are at least 70% accurate, sometimes fairly close, but you can't complain. They are computers that are fixed into certain rules that don't change according to individual language rules. Languages are complicated, each one on their own has their rules and their different names.
Take the words pig, hog or swine.
Wow that's a lot of names to call one little animal! Guess what? In Spanish the same animal has 6 different names that I know of!
And depending what Spanish speaking country you visit, the animal might have 2 or 3 more names!
You'd think embarrassed is translated as "embarazado" in Spanish. But unless science comes up with a male who can bare a child that translation is impossible. Only women can become pregnant, and it would be "embarazada". The correct translation for embarrassed into Spanish is "Abochornado" or "Abochornada" for a woman.
Take the word "Molestar" in Spanish. You'd think the correct translation is "Molest" but NO. Molestar in Spanish is "bother" nothing more. In Spanish the correct translation of Molest is "Acosar".
Which brings me back to the point that I am trying to make. You want 100% translation or at least 95%, hire a human translator, nothing is better than the brain of a human being thinking for the best way to translate what you need.
Maybe some day computers will be apt to translate languages the way they were meant to be. For now don't lose faith, they try their best to do a good job.
If one translator is not enough for you and the results are questionable, try two, or three. Some even have paragraph translations.
Check these translators out, I hope they help you: