Private corporations have a mission of generating wealth for the people that make it up, their stockholders. Unions have the same mission just for their members. Its pretty comparable.

Private corporations also have a pretty consistent thread of corruption, a similar example to the one you gave being HSBC laundering money for drug cartels. Again, pretty comparable.

This isn't to say corruption in unions hasn't been part of why they've declined, its almost definitely part of it. While a corrupt corporate leader generally creates more money for its stockholders a corrupt union rep often costs the rank-and-file members which can greatly sour their opinions of unions as a whole. But there's nothing inherent to unions with regard to corruption, people in power often take advantage of their position no matter what type of structure they have that power in, and the point that it appears, at least to some people, that unions are held to a different standard than other types of organizations is worthy of being discussed.