The Constitution does not contemplate impeachment for incompetence. Thomas Massie knows that, which is why his articles are grounded in alleged violations of statutes and the Constitution. That approach is legally appropriate.
The problem with most of his articles is that Hegseth was doing what Trump told him to do. That means, not just that he can plead the following orders defense, but that the rest of the GOP will rally around both him and Trump. As a practical matter, therefore, his articles are mostly doomed to fail.
The obvious exception is the allegation about attempting to prosecute Mark Kelly. That article has constitutional implications and cannot be tied directly to Trump. It also appeals to the self-interest of members of Congress. Its chances are much better.
I don’t know why Massie didn’t include an allegation about the mishandling of classified information, which would go beyond mere incompetence and cannot be attributed to Trump. If his articles fail, the Democrats would be wise to include it in a subsequent resolution after the end of the year.
Personally, I would say the real issue with Hegseth is his desire to politicize the military on Trump’s behalf, but I would acknowledge it might be difficult to prove that allegation.