Honesty, defined by my dictionary as "free of deceit and untruthfulness; sincere," is foundational to any good faith attempt to govern. The term 'good faith' is doing a lot of work here, and means that the person in the position of power is attempting to execute the responsibilities of their office to the best of their ability and as closely to the spirit of the law that establishes their office.
The advantages of being honest are therefore, legitimacy, positive accord with those governed, and security.
- Legitimacy, because you are only properly in an office to the extent that you were put there through no act of deception and execute your office in good faith;
- Positive accord, because whatever your relationship with the governed may be, everyone who is party to it knows where they stand and can agree as to the facts; and
- Security, because when you are honest you can never be caught in a lie - only an error.
There are no disadvantages, period, to being honest if you are occupying your office in good faith. From time to time someone may talk about being 'too honest,' but this is a misnomer. What they're actually objecting to is being too forthcoming, that is: too free with information that you needn't disclose. Honesty doesn't mean running your mouth off every chance you get, it simply means that when you open your mouth, what comes out is the truth to the best of your ability. Discretion is especially important when honesty is your watchword, but that's the closest to a disadvantage that you get and it can be absolutely devastating to be honest, but indiscrete.
Someone who occupies, or seeks, a political office in bad faith, however, does face serious disadvantages if they are too truthful, and this depends on the circumstances of the bad faith. If someone is engaged in corruption, criminality, or abuses of power, they will either have to lie about it or end up confessing to the misdeeds - deception becomes a necessity for the corruption to continue.
Even outside the realm of corruption, however, those who hold views that a majority of people find odious are strongly incentivized, structurally, to lie about their views in order to garner power, office, and favors. If they openly admit to their views they will be ostracized and isolated from power or political office.
In the case of political parties whose actual policy positions are odious to the public, there is strategic advantage to inventing crises that distract from the policy positions and insofar as their opponents have facts on their side, adding a miasma of uncertainty, doubt, or assumptions of bad faith to the whole political sphere allows them to operate under less effective scrutiny - and disturb the public less when discovered because, after all, "aren't all politicians automatically corrupt?"
Honesty serves best when the person holding power actually means to do well by the people governed. Dishonesty serves best when dealing with your enemies - and to a minority-rule government, the people are the enemy since the majority of them don't like the government, or wouldn't if the fully understood the truth of matters.