Existe algum corretor que realmente funcione?

Anyway, having one spelling-checker is better than none. Indeed it is designed to offer helpful hints and suggestions to your writing, not cover or proofread one´s text.
Many criticize, for example, some grammar rules that may leave the green-behind-the-ears writer confused. Namely, creating doubts with sentences where they can be acceptable (passibe voice, etc).
Overall, it can help you make fewer errors, just use it as a tool that help you with (invaluable) suggestions. In other words, it works and the one that uses it have to do his homework as well.

Yes, the human ones, but those also have perception problems and don´t work well when tired !