Competent people have a slight advantage over other employees. They tend to know what they are doing and get things done. Why is it then only a slight gain? There are no objective standards or measures of fairness. Competence is not always seen as a positive factor—it can be more a threat than anything else.
How can anyone consider virtues to be counter-productive? Easily. You just have to be the colleague who is not as career focused, prefers to take it easier, or who has stayed a long time in the company and has ‘earned’ the next promotion over the career rocket newcomer, or you just happen to be the boss of the competent employee who might start to turn into a threat to your position.
Mediocracy is the worst enemy of excellence. It cannot stand people who are good and devoted to their cause. Competent persons make the rest of the people look, well, average or less adorable to say the least. Contrast gives dynamics and variety but it also enables to set new standards and open doors for changes. Not everyone likes changes—it means that they have to learn something new and adjust to the circumstances. This might be an issue for you in case you have got your current position by favouritism or by means other than your skills and competence. Then everything is a threat that makes you vulnerable and open to environment where you may need to prove your worth and redo your magic. After all you have to stay put and hold your position since in the open market you may not get it again!
Competence is not the same as success. Some people attract the limelight and get the attention easily. Outside of the prime spot they may step over others, cheat and use dirty tricks, or hide their incompetence by taking credit from their subordinates. No doubt they are good at what they are doing but be aware since competence is only a matter of definition. This alternative career path does not certainly look very pretty even though the results may appear very impressive on the surface. Success, at all and any cost, may not be worth it after all.