1. The problem with criticizing monks to other people is it gives a bad name to the Sangha, even if you had legitimate grounds for the criticism. Just look at all the people who criticize Christianity and call all religion a scam because of some legitimately bad pastors. It damages the whole group. If you criticize monks it makes Buddhism look bad and turns people away from the Dhamma. People today have a lot of defilements and see the bad apples more than the good. If there are legitimate grounds for criticism it should be in a private to someone who can constructively correct this. If your intent is just to "spread the word" about this so-called bad monk rather than any kind of constructive or compassionate intention it is probably bad kamma.
2. You may not know the true reason behind the monks misbehavior if you see it in person. And if you just hear about the monk's actions online or on the news you may not know if its actually true or not. If the news or random people on online forums were misleading you or distorting the truth about this monk, you would end up criticizing a monk that may actually be a practicing ascetic or even a noble one. Most commonly people say that bad monks aren't true monks, which is the worst accusation if they actually are. Which is certainly bad and fairly heavy kamma.
As stated in the vissudhimagga.
Considering that this is bad, even if you didn't know they were actually a good monk, i would argue its best to restrain yourself from criticizing any monks to another person.82. Revilers of Noble Ones: being desirous of harm for Noble Ones consisting of
Buddhas, Paccekabuddhas, and disciples, and also of householders who are
stream-enterers, they revile them with the worst accusations or with denial of
their special qualities (see Ud 44 and MN 12); they abuse and upbraid them, is
what is meant.
83. Herein, it should be understood that when they say, “They have no
asceticism, they are not ascetics,” they revile them with the worst accusation;
and when they say, “They have no jhána or liberation or path of fruition, etc.,”
they revile them with denial of their special qualities. And whether done
knowingly or unknowingly it is in either case reviling of Noble Ones; it is weighty
kamma resembling that of immediate result, and it is an obstacle both to heaven
and to the path. But it is remediable