Although dishonest individuals do not always experience immediate repercussions for their unethical behavior, there are a number of benefits to having the self-discipline and courage to search for an honest career that is truly helps other people because the development of ethical habits early in one's professional career is likely to pay off later in life.
There can be significant long-term consequences to dishonest behavior, as shown by an increasing number of retractions of papers from scientific journals and a noticeable presence of scientific misconduct in the news. For example, Anil Potti resigned from Duke University after it was revealed that he published forged results and included inaccurate information in his CV (such as falsely claiming that he was a Rhodes Scholar). However, there are also a number less drastic consequences that do not involve formal punishment for bad behavior.
Embellishing results early in one's career can make downstream research more difficult. For example, inaccurate predictions will make it difficult to get positive results from follow-up experiments to validate a preliminary hypothesis. Also, many scientific disciplines offer rotations for graduate students, and it will be more difficult to recruit top-notch graduate students if other labs can offer more interesting projects with a better experimental design. It is difficult to maintain a steady stream of publications in a lab with little or no competent personnel.
Although scientists who forge results on a regular basis may not necessarily have to worry about downstream analysis (because all of their results are false anyways), individuals who make false claims on a regular basis are more likely to be caught by others attempting to verify important results.
Networking and social interactions will also be more difficult for individuals with a reputation for behaving dishonestly. Even if the general public is not aware of a person's reputation, individuals who behave unethically on a regular basis will probably have difficulties developing a close network of friends.
As my grandmother used to say, the common saying shouldn't be "practice makes perfect" but rather "practice makes permanent" because all habits (good or bad) are difficult to break. If aspiring scientists start engaging in unethical conduct, then it will become increasingly hard to break those habits at a later stage in their career.