The CAPTCHA system tested so far was relatively straightforward to solve—the black font color meant the text could easily be distinguished from the background, and additionally, the text was level and did not need to be rotated for Tesseract to interpret it accurately. Often, you will find websites using simple custom CAPTCHA systems similar to this, and in these cases, an OCR solution is practical. However, if a website uses a more complex system, such as Google's reCAPTCHA, OCR will take a lot more effort and may not be practical. Here are some more complex CAPTCHA images from around the web:
In these examples, the text is placed at different angles and with different fonts and colors, so a lot more work needs to be done to clean the image before OCR is practical. They are also somewhat difficult for people to interpret, particularly for those with vision disabilities.